Skip validating draw buffers when RASTERIZER_DISCARD is enabled. Bug: 962555, 962636 Change-Id: I4a22f83689252b7b7e51bbb3fc80e8f0d601a43f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1610701 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Shrek Shao <shrekshao@google.com>