gl: Add context options

gl: Add context options

Users of the GdkGLContext API should be allowed to set properties on the
shim GdkGLContext instance prior to realization, so that the
backend-specific implementation can use the value of those properties
when creating the windowing system specific resources.

The main three options are:

 • a major/minor version tuple, to request a specific GL version
 • a debug bit, to request a "debug context", which provides additional
   validation and run time checking
 • a forward compatibility bit, to request a context that does not
   have deprecated functionality

See also:
 - https://www.opengl.org/registry/specs/ARB/glx_create_context.txt

https://bugzilla.gnome.org/show_bug.cgi?id=741946