lowp: add constant_color, swap, move_dst_src

This is enough for us to do some really simple draws.
Also add some debug tools to help prioritize porting.

Change-Id: I334f8fd2133be1aeec3f3406371a81aa6c184776
Reviewed-on: https://skia-review.googlesource.com/18597
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>
4 files changed