When a buffer is mapped writable by the host, we need to
track its final contents, and then restore them during replay.

This implementation just blindly captures the whole mapped
range if writable, and can be optimized in the future.

This change renders the moving billboards in Manhattan correctly.

Test: First 500 frames of Manhattan and TRex
Bug: angleproject:4091
Change-Id: I5f7775235f569efb264bde679fd3045d7012c622
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2053512
Commit-Queue: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>