Fixed compilation on GLES
Switch MacOS to OpenTKGamePlatform
Destroy the GameWindow before shutting down OpenTK
Fix potential crash on shutdown

Releasing UI resources on a non-UI thread will lead to a crash on Mac
OS X. The user must explicitly call Game.Dispose() on the UI thread, or
risk a resource leak.
Remove ClearKeyCacheState() call

OpenTKGameWindow automatically clears the key state when a window loses
focus.
Removed MonoMac MacOS implementation
Respect DPI scaling