Fix x11 window size calculations (rust-windowing#402)

The fix for returning accurate window position lead to a regression
computing inner size in pixels. This commit resolves that by getting
inner size from the window ID winit caches and still resolving position
by climbing the window hierarchy.

Resolves rust-windowing#398

src/platform/linux/x11/window.rs

@@ -533,6 +533,17 @@ impl Window2 {
             let mut border: libc::c_uint = mem::uninitialized();
             let mut depth: libc::c_uint = mem::uninitialized();
 
+            // Get non-positioning data from winit window
+            if (self.x.display.xlib.XGetGeometry)(self.x.display.display, self.x.window,
+                &mut root, &mut x, &mut y, &mut width, &mut height,
+                &mut border, &mut depth) == 0
+            {
+                return None;
+            }
+
+            let width_out = width;
+            let height_out = height;
+            let border_out = border;
 
             // Some window managers like i3wm will actually nest application
             // windows (like those opened by winit) within other windows to, for
@@ -562,7 +573,7 @@ impl Window2 {
                 return None;
             }
 
-            Some((x as i32, y as i32, width as u32, height as u32, border as u32))
+            Some((x as i32, y as i32, width_out as u32, height_out as u32, border_out as u32))
         }
     }