Workaround for Intel 6xxx clear to opaque black bug

Bug: skia:
Change-Id: Id5e29b483c2b6f698219abfc5bbb2d858c4fc117
Reviewed-on: https://skia-review.googlesource.com/16427
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
5 files changed