Metal: Support RGBA IOSurface pbuffer. Currently only BGRA IOSurface is supported by ANGLE. This CL adds RGBA IOSurface support to Metal backend. - Added GL_RGBX8_ANGLE & GL_UNSIGNED_BYTE attributes combo to specify that the IOSurface is RGBX. Only supported on Metal backend for now. - Added GL_RGBA & GL_UNSIGNED_BYTE combo to specify that the IOSurface is RGBA. Only supported on Metal backend for now. Bug: angleproject:8350 Change-Id: I0e05762870c9c034bca78e8989aedf346406df57 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4874484 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Quyen Le <lehoangquyen@chromium.org>