Improve image update perf by re-using a MemoryBuffer. Instead of allocating a new MemoryBuffer every frame, we can store one scratch buffer in the Renderer, and occasionally re-create it to ensure we aren't stuck using the maximum amount of memory. BUG=angle:842 Change-Id: Id7c1912921ed8f84b151413453c3268d853352db Reviewed-on: https://chromium-review.googlesource.com/230861 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Bruce Dawson <brucedawson@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>