Validate that fragment shader output matches the draw buffer type. TEST=conformance2/rendering/fs-color-type-mismatch-color-buffer-type BUG=angleproject:1688 Change-Id: I17848baf40b6d32b5adc1458fe2369b850164da3 Reviewed-on: https://chromium-review.googlesource.com/518246 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Frank Henigman <fjhenigman@chromium.org>