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