Add a test for glClearBuffers of render targets of same FBO This test is to ensure the upcoming Vulkan implementation can coalesce all the calls into a single render pass clear. Bug: angleproject:2361 Change-Id: I816907b3512715c1d3217689413b6c8e9842a37a Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1544973 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org>