Restrict color writemasks for RGB9_E5 color buffers Per-channel write operations to shared exponent color buffers are loosely defined and may cause driver validation errors. Restricted the set of allowed color writemasks for RGB9_E5 color buffers so that RGB channels must be either all enabled or all disabled. Added a Metal-specific adjustment to ignore alpha writemask for RGB9_E5 color buffers. Removed an unused function from RenderPipelineColorAttachmentDesc. Bug: angleproject:8043 Change-Id: I902c3b70ddc6d8e65069d98a4a02a82122f413a5 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4685566 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>