meta: Add support for doing multisample resolves.

meta: Add support for doing multisample resolves.

Note that this doesn't handle GL_EXT_multisample_scaled_blit yet.  The
i965 code for that extension bakes in knowledge of the sample positions
(well, knowledge of the sample positions aligned to a lower-resolution
grid), which we would have to do at runtime somehow for meta.

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