Vulkan: Suppress layer warnings about unbound outputs. This warning could pop up whenever an OpenGL app would write to a particular output and not bind an attachment. This is valid in OpenGL. Also it could happen when writing to gl_FragData instead of gl_FragColor. Since it's hard to fix every usage we can just suppress the warning. Also adds a way to change test platform warnings into errors. Bug: angleproject:2866 Change-Id: I9793f58121ac848d74d6b0131e79ebab2c70f45c Reviewed-on: https://chromium-review.googlesource.com/c/1462057 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>