Instead of doing a masked clear when a texture has mismatched channel
counts, set the clear color to the default for the missing channels and do
a regular clear.

This catches es3proto up to master in terms of clear performance.

BUG=angle:557

Change-Id: I4f16976154af7735f5bb2b0b17adb3a264872ec0
Reviewed-on: https://chromium-review.googlesource.com/186611Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>