mesa: only use fallback texture when using shaders, not fixed-function (take two)

mesa: only use fallback texture when using shaders, not fixed-function (take two)

The semantics are a little different for shaders vs. fixed-function when
trying to use an incomplete texture.  The fallback texture returning
(0,0,0,1) should only be used with shaders.  For fixed function, the texture
unit is truly disabled/ignored.

Fixes glean fbo test regression.
(cherry picked from commit 01e16befd4809e3e93ee7855a4a5d7df9fe010d9)
(cherry picked from commit 51325f44d1e7e62b47795a79f8038d10dc5bc30b)
[anholt: squashed these two together from master, skipping the mess in between]