This commit does a fairly large cleanup of blend descriptors, although
there should not be any functional changes. In particular, we split
apart the Midgard and Bifrost blend descriptors, since they are
radically different. From there, we can identify that the Midgard
descriptor as previously written was really two render targets'
descriptors stuck together. From this observation, we split the Midgard
descriptor into what a single RT actually needs. This enables us to
correctly dump blending configuration for MRT samples on Midgard. It
also allows the Midgard and Bifrost blend code to peacefully coexist,
with runtime selection rather than a #ifdef. So, as a bonus, this will
help the future Bifrost effort, eliminating one major source of
compile-time architectural divergence.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>