Metal: Skip disabled draw buffers When creating render pass descriptors, do not assign textures to disabled color attachments. When creating pipeline descriptors, reset pixel formats of disabled color attachments. Exit early when MTLRenderCommandEncoder is not created. Added: * DrawBuffersTest.None * DrawBuffersTest.NoneWithDepth * DrawBuffersTest.NoneWithStencil * DrawBuffersTestES3.DrawWithDisabledIncompatibleAttachment Fixes: * conformance2/rendering/fs-color-type-mismatch-color-buffer-type.html Bug: angleproject:6430 Change-Id: I7f650c761f757985b027388c350c01340a83fd51 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4003180 Reviewed-by: Kenneth Russell <kbr@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>