Color renderbuffers are similar to multisampled-render-to-texture
textures, but much simpler.  The same mechanism is used to implement
them.

Bug: angleproject:4836
Change-Id: I298529c9fd1b03e78b1e37cdbe595e66166ee130
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2412847
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>