PBOs aren't generally faster any more.
NPOT textures are widely supported.
Giving control over to Gtk while waiting for a swap doesn't really work
that well, so the sync control extension options are removed.
This means the OpenGL and Vulkan backends are in parity with regard to
options.
GTK: Remove support for GTK+ 2.0.
GTK 3 is stable and widespread enough now.
GTK: Rearrange headers to eliminate gtk_s9xcore.h
Gtk: Initial gtkmm conversion work.
Gtk: More gtkmm conversion and bug fixing.
Gtk: More gtkmm fixes.
Gtk: More Fixes
OpenGL no longer creates a second window.
Accelerators are fixed.
Gtk: More fixes
Removed GLX context dependency on Gtk.
Gtk: Fix formatting.
Gtk: Remove a #pragma once
