Add separate dirty bits for Pixel and Fragment uniforms. Also use a single UpdateSubresource call to update the buffer. Should improve performance on some benchmarks. BUG=angleproject:1390 Change-Id: I70d54d86d3d3beb0e2caee86338ee03081070ac8 Reviewed-on: https://chromium-review.googlesource.com/663895 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>