Update ANGLE's default framebuffer implementation to support reading
and writing to different surfaces within the same framebuffer.

Bug: angleproject:2620
Test: EGLSurfaceTest[3]
Change-Id: I4b1ea04ca87a751f80cf190bf3adec148fc4fce3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1744746
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>