API: Add gdk_display_create_gl_context()

API: Add gdk_display_create_gl_context()

This is an alternative to gdk_surface_create_gl_context() when the
context is meant to only draw to textures.

This is useful in the testsuite or in GStreamer or with GLArea,
basically whenever we want to do GL stuff but don't need to actually
draw anything on screen.

A bunch of code will need to be updated to deal with context->surface
being NULL.