Avoid special shader for opaque alpha when drawing simple rectangles.

It's a wasted shader compiler and also seems to cause a rendering bug
in Windows Chrome on ANGLE.

Bug: chromium:942565
Change-Id: I56373ac596d4d1548d6c86cda7127bf12190a0ae
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/203383
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
3 files changed