Add support for xcb

Due to XCB and Xlib compability, we can take a shortcut and use X11's
underlying xcb_connection. This way, a complete XCB backend implementation can
be avoided.

src/api/x11/ffi.rs

@@ -4,4 +4,5 @@ pub use x11_dl::xf86vmode::*;
 pub use x11_dl::xlib::*;
 pub use x11_dl::xinput::*;
 pub use x11_dl::xinput2::*;
+pub use x11_dl::xlib_xcb::*;
 pub use x11_dl::error::OpenError;

src/api/x11/window.rs

@@ -694,6 +694,12 @@ impl Window {
         self.x.window as *mut libc::c_void
     }
 
+    pub fn get_xcb_connection(&self) -> *mut libc::c_void {
+        unsafe {
+            (self.x.display.xlib_xcb.XGetXCBConnection)(self.get_xlib_display() as *mut _) as *mut _
+        }
+    }
+
     #[inline]
     pub fn set_window_resize_callback(&mut self, _: Option<fn(u32, u32)>) {
     }

src/api/x11/xdisplay.rs

@@ -15,6 +15,7 @@ pub struct XConnection {
     pub xf86vmode: ffi::Xf86vmode,
     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>>,
 }
@@ -31,6 +32,7 @@ impl XConnection {
         let xcursor = try!(ffi::Xcursor::open());
         let xf86vmode = try!(ffi::Xf86vmode::open());
         let xinput2 = try!(ffi::XInput2::open());
+        let xlib_xcb = try!(ffi::Xlib_xcb::open());
 
         unsafe { (xlib.XInitThreads)() };
         unsafe { (xlib.XSetErrorHandler)(error_handler) };
@@ -49,6 +51,7 @@ impl XConnection {
             xf86vmode: xf86vmode,
             xcursor: xcursor,
             xinput2: xinput2,
+            xlib_xcb: xlib_xcb,
             display: display,
             latest_error: Mutex::new(None),
         })