When the scissor region given was completely outside the renderArea,
we were calling vkCmdClearAttachments with the unmodified rectangle.

Instead, we just need to actually skip the clear call since there is
nothing to clear if we have a scissor region outside the render area.

Bug: angleproject:2484

Change-Id: If2c7c5b91e07081f1d2e4f4d8d13f8c8a842ea0b
Reviewed-on: https://chromium-review.googlesource.com/1014958
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>