Vulkan: Use render pass loadOp for scissored clears 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>