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/util/randr.rs

@@ -0,0 +1,111 @@
+use std::{env, slice};
+use std::str::FromStr;
+
+use validate_hidpi_factor;
+use super::*;
+
+pub fn calc_dpi_factor(
+    (width_px, height_px): (u32, u32),
+    (width_mm, height_mm): (u64, u64),
+) -> f64 {
+    // Override DPI if `WINIT_HIDPI_FACTOR` variable is set
+    let dpi_override = env::var("WINIT_HIDPI_FACTOR")
+        .ok()
+        .and_then(|var| f64::from_str(&var).ok());
+    if let Some(dpi_override) = dpi_override {
+        if !validate_hidpi_factor(dpi_override) {
+            panic!(
+                "`WINIT_HIDPI_FACTOR` invalid; DPI factors must be normal floats greater than 0. Got `{}`",
+                dpi_override,
+            );
+        }
+        return dpi_override;
+    }
+
+    // See http://xpra.org/trac/ticket/728 for more information.
+    if width_mm == 0 || width_mm == 0 {
+        warn!("XRandR reported that the display's 0mm in size, which is certifiably insane");
+        return 1.0;
+    }
+
+    let ppmm = (
+        (width_px as f64 * height_px as f64) / (width_mm as f64 * height_mm as f64)
+    ).sqrt();
+    // Quantize 1/12 step size
+    let dpi_factor = ((ppmm * (12.0 * 25.4 / 96.0)).round() / 12.0).max(1.0);
+    assert!(validate_hidpi_factor(dpi_factor));
+    dpi_factor
+}
+
+pub enum MonitorRepr {
+    Monitor(*mut ffi::XRRMonitorInfo),
+    Crtc(*mut ffi::XRRCrtcInfo),
+}
+
+impl MonitorRepr {
+    pub unsafe fn get_output(&self) -> ffi::RROutput {
+        match *self {
+            // Same member names, but different locations within the struct...
+            MonitorRepr::Monitor(monitor) => *((*monitor).outputs.offset(0)),
+            MonitorRepr::Crtc(crtc) => *((*crtc).outputs.offset(0)),
+        }
+    }
+
+    pub unsafe fn get_dimensions(&self) -> (u32, u32) {
+        match *self {
+            MonitorRepr::Monitor(monitor) => ((*monitor).width as u32, (*monitor).height as u32),
+            MonitorRepr::Crtc(crtc) => ((*crtc).width as u32, (*crtc).height as u32),
+        }
+    }
+
+    pub unsafe fn get_position(&self) -> (i32, i32) {
+        match *self {
+            MonitorRepr::Monitor(monitor) => ((*monitor).x as i32, (*monitor).y as i32),
+            MonitorRepr::Crtc(crtc) => ((*crtc).x as i32, (*crtc).y as i32),
+        }
+    }
+}
+
+impl From<*mut ffi::XRRMonitorInfo> for MonitorRepr {
+    fn from(monitor: *mut ffi::XRRMonitorInfo) -> Self {
+        MonitorRepr::Monitor(monitor)
+    }
+}
+
+impl From<*mut ffi::XRRCrtcInfo> for MonitorRepr {
+    fn from(crtc: *mut ffi::XRRCrtcInfo) -> Self {
+        MonitorRepr::Crtc(crtc)
+    }
+}
+
+impl XConnection {
+    pub unsafe fn get_output_info(
+        &self,
+        resources: *mut ffi::XRRScreenResources,
+        repr: &MonitorRepr,
+    ) -> Option<(String, f64)> {
+        let output_info = (self.xrandr.XRRGetOutputInfo)(
+            self.display,
+            resources,
+            repr.get_output(),
+        );
+        if output_info.is_null() {
+            // When calling `XRRGetOutputInfo` on a virtual monitor (versus a physical display)
+            // it's possible for it to return null.
+            // https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=816596
+            let _ = self.check_errors(); // discard `BadRROutput` error
+            return None;
+        }
+        let name_slice = slice::from_raw_parts(
+            (*output_info).name as *mut u8,
+            (*output_info).nameLen as usize,
+        );
+        let name = String::from_utf8_lossy(name_slice).into();
+        let hidpi_factor = calc_dpi_factor(
+            repr.get_dimensions(),
+            ((*output_info).mm_width as u64, (*output_info).mm_height as u64),
+        );
+        (self.xrandr.XRRFreeOutputInfo)(output_info);
+        Some((name, hidpi_factor))
+    }
+}