Metal: Disable blending for masked out attachments Avoid pipeline failure when the following conditions are simultaneously true for the same color attachment: * Dual-source blending is enabled * All color channels are masked out * There is no bound fragment output Bug: angleproject:8015 Change-Id: Ie60f4f6dff5a9023677ea9b581e920b0e1e53532 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4836558 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>