mesa: Split the shader texture update logic from fixed function.

mesa: Split the shader texture update logic from fixed function.

I want to avoid walking the entire long array texture image units, but the
obvious way to do so means walking program samplers, and thus hitting the
units in a random order.

This change replaces the previous behavior of only setting up the fallback
texture for a fragment shader with setting up the fallback texture for any
shader that's missing a complete texture of the right target in its unit.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>