At this point, every clear is done through render pass loadOp, except
masked color or stencil clears.  The only fallback is clearWithDraw,
that can clear both color and stencil at the same time.

Bug: angleproject:2361
Change-Id: I805fc12475e832ad2f573f665cdfeb766e61a6d0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1553740
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tobin Ehlis <tobine@google.com>