Covers a bug Skia's implementation of unresolve uncovered.  ANGLE's
implementation is slightly different (in that the unresolve pass is full
screen), so it's not affected by this bug.

Bug: angleproject:4836
Change-Id: Ibc8010b0b5ef6912a45fd5f2612fb8b619e4407e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2582707Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>