Skip component type validation of non-existent draw buffers The default framebuffer is initialized with just one draw buffer slot so WebGL-specific validation causes an assertion when clearing not present draw buffers. Bug: angleproject:2831 Change-Id: I23ceb3f31dd8bae005fe8497cf20c49880f37938 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4683451 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>