Event Loop 2.0 API and Windows implementation (#638)

* Rename EventsLoop and associated types to EventLoop

* Rename WindowEvent::Refresh to WindowEvent::Redraw

* Remove second thread from win32 backend

* Update run_forever to hijack thread

* Replace windows Mutex with parking_lot Mutex

* Implement new ControlFlow and associated events

* Add StartCause::Init support, timer example

* Add ability to send custom user events

* Fully invert windows control flow so win32 calls into winit's callback

* Add request_redraw

* Rename platform to platform_impl

* Rename os to platform, add Ext trait postfixes

* Add platform::desktop module with EventLoopExt::run_return

* Re-organize into module structure

* Improve documentation

* Small changes to examples

* Improve docs for run and run_return

* Change instances of "events_loop" to "event_loop"

* Rename MonitorId to MonitorHandle

* Add CHANGELOG entry

* Improve WaitUntil timer precision

* When SendEvent is called during event closure, buffer events

* Fix resize lag when waiting in some situations

* Update send test and errors that broke some examples/APIs

* Improve clarity/fix typos in docs

* Fix unreachable panic after setting ControlFlow to Poll during some RedrawRequested events.

* Fix crash when running in release mode

* Remove crossbeam dependency and make drop events work again

* Remove serde implementations from ControlFlow

* Fix 1.24.1 build

* Fix freeze when setting decorations

* Replace &EventLoop in callback with &EventLoopWindowTarget

* Document and implement Debug for EventLoopWindowTarget

* Fix some deadlocks that could occur when changing window state

* Fix thread executor not executing closure when called from non-loop thread

* Fix buffered events not getting dispatched

* Fix crash with runner refcell not getting dropped

* Address review feedback

* Fix CHANGELOG typo

* Catch panics in user callback

src/platform_impl/linux/x11/xdisplay.rs

@@ -0,0 +1,155 @@
+use std::ptr;
+use std::fmt;
+use std::error::Error;
+
+use libc;
+use parking_lot::Mutex;
+
+use super::ffi;
+
+/// A connection to an X server.
+pub struct XConnection {
+    pub xlib: ffi::Xlib,
+    /// Exposes XRandR functions from version < 1.5
+    pub xrandr: ffi::Xrandr_2_2_0,
+    /// Exposes XRandR functions from version = 1.5
+    pub xrandr_1_5: Option<ffi::Xrandr>,
+    pub xcursor: ffi::Xcursor,
+    pub xinput2: ffi::XInput2,
+    pub xlib_xcb: ffi::Xlib_xcb,
+    pub display: *mut ffi::Display,
+    pub latest_error: Mutex<Option<XError>>,
+}
+
+unsafe impl Send for XConnection {}
+unsafe impl Sync for XConnection {}
+
+pub type XErrorHandler = Option<unsafe extern fn(*mut ffi::Display, *mut ffi::XErrorEvent) -> libc::c_int>;
+
+impl XConnection {
+    pub fn new(error_handler: XErrorHandler) -> Result<XConnection, XNotSupported> {
+        // opening the libraries
+        let xlib = ffi::Xlib::open()?;
+        let xcursor = ffi::Xcursor::open()?;
+        let xrandr = ffi::Xrandr_2_2_0::open()?;
+        let xrandr_1_5 = ffi::Xrandr::open().ok();
+        let xinput2 = ffi::XInput2::open()?;
+        let xlib_xcb = ffi::Xlib_xcb::open()?;
+
+        unsafe { (xlib.XInitThreads)() };
+        unsafe { (xlib.XSetErrorHandler)(error_handler) };
+
+        // calling XOpenDisplay
+        let display = unsafe {
+            let display = (xlib.XOpenDisplay)(ptr::null());
+            if display.is_null() {
+                return Err(XNotSupported::XOpenDisplayFailed);
+            }
+            display
+        };
+
+        Ok(XConnection {
+            xlib,
+            xrandr,
+            xrandr_1_5,
+            xcursor,
+            xinput2,
+            xlib_xcb,
+            display,
+            latest_error: Mutex::new(None),
+        })
+    }
+
+    /// Checks whether an error has been triggered by the previous function calls.
+    #[inline]
+    pub fn check_errors(&self) -> Result<(), XError> {
+        let error = self.latest_error.lock().take();
+        if let Some(error) = error {
+            Err(error)
+        } else {
+            Ok(())
+        }
+    }
+
+    /// Ignores any previous error.
+    #[inline]
+    pub fn ignore_error(&self) {
+        *self.latest_error.lock() = None;
+    }
+}
+
+impl fmt::Debug for XConnection {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.display.fmt(f)
+    }
+}
+
+impl Drop for XConnection {
+    #[inline]
+    fn drop(&mut self) {
+        unsafe { (self.xlib.XCloseDisplay)(self.display) };
+    }
+}
+
+/// Error triggered by xlib.
+#[derive(Debug, Clone)]
+pub struct XError {
+    pub description: String,
+    pub error_code: u8,
+    pub request_code: u8,
+    pub minor_code: u8,
+}
+
+impl Error for XError {
+    #[inline]
+    fn description(&self) -> &str {
+        &self.description
+    }
+}
+
+impl fmt::Display for XError {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        write!(formatter, "X error: {} (code: {}, request code: {}, minor code: {})",
+               self.description, self.error_code, self.request_code, self.minor_code)
+    }
+}
+
+/// Error returned if this system doesn't have XLib or can't create an X connection.
+#[derive(Clone, Debug)]
+pub enum XNotSupported {
+    /// Failed to load one or several shared libraries.
+    LibraryOpenError(ffi::OpenError),
+    /// Connecting to the X server with `XOpenDisplay` failed.
+    XOpenDisplayFailed,     // TODO: add better message
+}
+
+impl From<ffi::OpenError> for XNotSupported {
+    #[inline]
+    fn from(err: ffi::OpenError) -> XNotSupported {
+        XNotSupported::LibraryOpenError(err)
+    }
+}
+
+impl Error for XNotSupported {
+    #[inline]
+    fn description(&self) -> &str {
+        match *self {
+            XNotSupported::LibraryOpenError(_) => "Failed to load one of xlib's shared libraries",
+            XNotSupported::XOpenDisplayFailed => "Failed to open connection to X server",
+        }
+    }
+
+    #[inline]
+    fn cause(&self) -> Option<&Error> {
+        match *self {
+            XNotSupported::LibraryOpenError(ref err) => Some(err),
+            _ => None
+        }
+    }
+}
+
+impl fmt::Display for XNotSupported {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        formatter.write_str(self.description())
+    }
+}