- Generalize cmdClearAttachments to be able to clear
depth/stencil/color individually.
- The clear of the stencil part is implemented, but cant be tested
until stencil state support is implemented in the next commit.

Bug: angleproject:2427

Change-Id: Ib1fc7f66b2bd232761d374e217812605d3ff9152
Reviewed-on: https://chromium-review.googlesource.com/992874Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Luc Ferron <lucferron@chromium.org>