Unit test to demonstrate Galaxy S6 MSAA readPixels bug

After lots of tinkering, this is the smallest test case I've
found to reproduce the bug. Frequently, bitmap b4 will be
cleared to blue, but not contain the oval drawn right before.
Bitmap b5 will contain the oval, so the diff of the two
bitmaps will detect the error.

There may be something else that can be removed, but I
haven't found it.

Interesting note:

In the original version, all surfaces were cleared to
black. When I started trying to change the clear colors to
figure out if we were getting an old surface, the bug went
away. In particular, the first clear color is irrelevant,
but the last three clears must all be the same color. If
any are different, the bug doesn't occur.

Bug: skia:6653
Change-Id: Iacafcc140b60594fab208e82987b0f37416975f3
Reviewed-on: https://skia-review.googlesource.com/19817
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
4 files changed