Add a GM that reproduces layout test failures with my new xfermode code.

Inspired by https://storage.googleapis.com/chromium-layout-test-archives/linux_blink_rel/69169/layout-test-results/results.html

I think the root cause is overflow.

Also, adds tests for Sk16b::operator<().  It wasn't wrong, but it was suspect
(used in all three of these xfermode implementations) and so it's best to have
tests.

BUG=skia:

Review URL: https://codereview.chromium.org/1228393006
2 files changed