Fix broadcasting gl_FragColor at the end of main Previously, the broadcast step would not get run in case the main() function returned before reaching its end. Now the broadcast step is put in a separate function that wraps main() if needed, so that it gets run even if the main() function in the original shader source returns in the middle. DrawBuffersTest is refactored to use ANGLETest::drawQuad() instead of calling glDrawArrays directly. BUG=angleproject:2109 TEST=WebGL conformance tests, angle_end2end_tests Change-Id: Id5f05094e816df03bc9c8ca62b60de914072682c Reviewed-on: https://chromium-review.googlesource.com/574597 Reviewed-by: Zhenyao Mo <zmo@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>