gdkgl: Use vfunc For Uploading Textures

As the alignments, strides and image formats may be different across
platforms, make the texture upload a vfunc to allow backends to override
the GL commands for uploading textures for the software implementation for
gdk_gl_texture_from_surface(), if necessary.

Suggested by Alex to avoid copying non-trivial portions of code which would
then add maintainenace burden.

https://bugzilla.gnome.org/show_bug.cgi?id=740795
Add OpenGL Support for the GDK Windows Backend

This adds support for OpenGL to the GDK Windows backend using the WGL API
calls, which enables programs that uses the GTK+ GLArea widgets to work on
Windows as well.

This also adds a simple utility function to query for the version of OpenGL
that is supported by the Windows system, like the one provided by the X11
backend.

Many thanks to Alex (and Emmanuele, who started the OpenGL integration in
GTK+) who offered advice and help along the way, as well as the X11 and
Wayland backend for this work to refer to and to model upon.

https://bugzilla.gnome.org/show_bug.cgi?id=740795