mesa: rewrite accum buffer support

mesa: rewrite accum buffer support

Implemented in terms of renderbuffer mapping/unmapping and format
packing/unpacking functions.

The swrast and state tracker code for implementing accumulation are
unused and will be removed in the next commit.

v2: don't use memcpy() in _mesa_clear_accum_buffer()
v3: don't allocate MAX_WIDTH arrays, be more careful with mapping flags

Reviewed-by: Eric Anholt <eric@anholt.net>