- Skip clears if an empty scissor rect is set
- Use rastStates to apply scissoring instead of recomputing the scissored
positions and updating the VB and updating the VB for every clear
- Use ComPtrs for VB and RasterizerStates

BUG=angleproject:1934

Change-Id: Ib4448d8568b465df5de57607be4985095f6bd014
Reviewed-on: https://chromium-review.googlesource.com/453881
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>