Enable CopyTex(Sub)Image default FB on Mesa/GLX. Allow CopyTexImageTest.SubDefaultFramebuffer and CopyTexImageTest.DefaultFramebuffer to work with Mesa and GLX back end by doing a clear before reading the framebuffer. BUG=angleproject:2027 Change-Id: Ibb10369e3f5aceb60511fbed1d67a28132734b29 Reviewed-on: https://chromium-review.googlesource.com/507952 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>