Adds serialization of depth/stencil buffer contents
Currently, readPixels on color buffers uses the gles front-end format,
which can be different from the implementation format used in the
backend. This can cause seg fault crashes when the number of bytes in
a pixel differs between the two format. Changes so that readPixels for
color buffers always use the backend format.
Bug: angleproject:4817
Change-Id: I62d5deb1320ea38ee46143b2aa97f372ba01baeb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2284451Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Manh Nguyen <nguyenmh@google.com>
