st/mesa: add option to choose a texture format that we won't render to.

st/mesa: add option to choose a texture format that we won't render to.

We need a texture to put the drawpixels stuff into, an S8 texture is less
memory/bandwidth than the 32-bit X24S8, but we might not be able to render
directly to an S8, so this lets us specify we won't be rendering to this
texture.