glxsimple: Introduce an inter-frame texture dependency to exercise trim code

The previous glxsimple program was too simple as each frame was
entirely self-contained, (textures were created, initialized, and
referenced only within single frames).

For more realistic trimming, we create and initialize a texture in one
frame, then reference that texture in a later frame. This is a better
test to ensure that when trimming the first frame the texture creation
and initialization is not discarded.