Fix alpha threshold image filter GPU implementation

BUG=skia:7760
Change-Id: I674d705ce637c405361ecf45ea8604d1606f7af7
Reviewed-on: https://skia-review.googlesource.com/120203
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
1 file changed