Serialize d/s buffers + use color buffers' impl format for readPixels 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/+/2284451 Reviewed-by: Cody Northrop <cnorthrop@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Manh Nguyen <nguyenmh@google.com>