D3D11: Ignore sample mask and A2C for single-sampled rendering Also fixed alpha-to-coverage for single-sampled rendering and simplified sample coverage code in the Vulkan backend. Fixed: angleproject:8102 Change-Id: Ieea03dfdc13c6105ddf916dca6d0fea593eb3a62 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4455508 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>