gl-renderer: require GL_EXT_unpack_subimage

Require GL_EXT_unpack_subimage unconditionally in GL-renderer. Without this extension, it would take considerable effort in GL-renderer to handle correctly images that contain row padding, either as a temporary copy to remove padding or doing SubImage updates row by row.

I would guess that this path has gone long completely untested, and if it was exercised, the rows never had padding thanks to 32-bit pixel formats. Instead of writing tests to poke the corner cases and fixing it, remove it.

This will make it easier to fix other problems in GL-renderer in this area in the future - one less path to consider and many restrictions in GL API gone.