Vulkan: fix non-float clear with draw 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>