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/ui/gl/circular_buffer.cc

@@ -11,7 +11,6 @@
 
 #include "xenia/base/assert.h"
 #include "xenia/base/math.h"
-#include "xenia/gpu/gl4/gl4_gpu_flags.h"
 
 namespace xe {
 namespace ui {