This uses a trick by comparing the prior to the updated FB description
to filter out rendundant changes. Will allow the front-end syncState to
sometimes noisily report state changes when there aren't any in the
back-end. Ideally we wouldn't generate any updates when there aren't
real changes. However because Manhattan often transitons between a few
zero states it gets a bit difficult to determine in the front-end.

Also adds a no-op for redundant samples changes in the front-end.

Fixes a regression in the number of RenderPasses in Manhattan from work
to support rendering feedback loops.

Bug: angleproject:4490
Change-Id: Ibf83d584107a4561da7ad47a2d3fb543e63e31f1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2112935Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>