Serializes renderbuffers' states stored on CPU.
Gets renderbuffers content from GPU then serialize them.

Adds renderbuffers serialization to serializeContext method so that
capture replay regresssion testing now compares the states of
renderbuffers too.

Bug: angleproject:4817
Change-Id: I537b11ee85decb14eea461ebbd62ce6ab0402f85
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2327173
Commit-Queue: Manh Nguyen <nguyenmh@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>