Turns out this is much faster than using dynamic state. When we
support multiple viewports it might be easier to use dynamic
state since we won't need to make an overly large pipeline
description. We could support both methods using a flag to
indicate the viewport and/or scissor regions are invalid.
Until then we can remove the pipeline and scissor dirty bits.

Improves perf by about 15% in the Vulkan VBO state change test.

Bug: angleproject:3013
Change-Id: I4572250626a9a0f0ca3451b17e8f0de186416cae
Reviewed-on: https://chromium-review.googlesource.com/c/1390359
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>