vc4: Add support for texel fetches from MSAA resources.

vc4: Add support for texel fetches from MSAA resources.

This is the core of ARB_texture_multisample.  Most of the piglit tests for
GL_ARB_texture_multisample require GL 3.0, but exposing support for this
lets us use the gallium blitter for multisample resolves.  We can
sometimes multisample resolve using just the RCL, but that requires that
the blit is 1:1, unflipped, and aligned to tile boundaries.