st/mesa: re-do binding flags in st_ChooseTextureFormat(), again

st/mesa: re-do binding flags in st_ChooseTextureFormat(), again

Try to specify render target bindings flags first.  If that fails, try
again with just sampler view binding.  Note that we try to create the
texture resource with render target binding flags later when we allocate
the texture.  Then, in FBO validation, we check if we can actually render
to the textures.  If that fails, we generate GL_FRAMEBUFFER_UNSUPPORTED_EXT.

Changes suggested by Jose.