Switch from GtkGLExt to internal OpenGL handling

GtkGLExt has many problems:
  - It's code is bloated and convoluted
  - It doesn't work on Win32
  - It doesn't work with gtk+-3.0
  - The last release was in early 2006

Using our own lets us fix these issues and should be easier to maintain
than trying to figure out the GtkGLExt mess, or waiting for someone else
to fix it.