viz: Add alpha_type param to SkiaOutputSurface::CreateSharedImage

This CL lets SkiaRenderer explicitly specify an alpha_type for its
render pass backings. This should make no visible difference in
behavior.

SkiaOutputSurface[Impl[OnGpu]]::CreateSharedImage is used for render
pass backing allocations. Normally, the alpha type of render pass
backings doesn't matter much when SkiaRenderer is doing the
compositing. However, on some platforms (i.e. Windows), the alpha type
of an overlay image has implications for performance in the system
compositor.

Bug: 1468570
Bug: 1434131
Change-Id: Ia4b3b11983c27c2884819077652d5f57ce38b185
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4605283
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Reviewed-by: Rafael Cintron <rafael.cintron@microsoft.com>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Commit-Queue: Michael Tang <tangm@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1202040}