Massive refactoring of xenia::ui and GL swap behavior.

This seems to dramatically improve most games (especially with
--vsync=false), though it may cause swap issues with others.
New code should be easier to port, and enables elemental-forms to be
drawn for any emulator UI.

src/xenia/gpu/graphics_system.cc

@@ -43,7 +43,7 @@ GraphicsSystem::GraphicsSystem(Emulator* emulator) : emulator_(emulator) {}
 GraphicsSystem::~GraphicsSystem() = default;
 
 X_STATUS GraphicsSystem::Setup(cpu::Processor* processor, ui::Loop* target_loop,
-                               ui::PlatformWindow* target_window) {
+                               ui::Window* target_window) {
   processor_ = processor;
   memory_ = processor->memory();
   target_loop_ = target_loop;