add paletted source support

For the moment, looks like we only need an 8-bit palette with an 8888
payload.

Some little updates to bench.c to keep it sane with palettes.

Bug: skia:8473
Change-Id: I4efffea3273b5565954c0d20a666979f11ea685b
Reviewed-on: https://skia-review.googlesource.com/c/163170
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
6 files changed