Instead of using one draw call that clears all attachments, multiple
draw calls are issued that clear a single attachment each.  This allows
us to have a manageable number of variations for the ImageClear.frag
shader, now that non-float format support is introduced.

Bug: angleproject:3187
Change-Id: Ic0c1067a396250bd80f31d00cad5a272acff8be8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1545523
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>