Implement EXT_discard_framebuffer for the GL backend. BUG=angleproject:1634 Change-Id: I3822b99b59d4653e4d9a2c1d3dd16734f2050fae Reviewed-on: https://chromium-review.googlesource.com/414437 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>