[graphite] Remove rgba8 colortype fallback for MakeBitmapProxyView

This means that SkImages::MakeTexture follows the same restrictions as
SkSurfaces::RenderTarget; if the compatible texture formats aren't
supported, then the SkImage will be null instead of magically changing
to a format and colortype that wasn't requested (triggering on-CPU
conversion during an upload or readback).

A behavior consequence is that now the grdawn configs will not draw
images that have a 565 or 4444 color type because there is no format
representation for those colortypes in WebGPU. This may be added as an
extension later (particularly around 565). Similarly, non-Apple Macs do
not support these formats, so they will stop rendering in grmtl.

A code consequence is that MakeBitmapProxyView doesn't have to play
games with applying the alpha type swizzle to an RGBA image. This was
already not the best thing to do because it would sample AAAA instead of
000A as expected, but additionally the SkImage's color type have been
RGBA8, so all the shader combination logic would not add the necessary
blends with the paint color (as it no longer appears alpha-only
according to its color type).

This alpha AAAA swizzling was really just patching up the sampling logic
that was used for handling R8 vs A8 single plane images used in the YUVA
shader. This CL updates that logic to detect an alpha-only swizzle or
format and move it to the red channel, and removes a similar AAAA
swizzle concat for YUVA from SkImage (vs. from proxies).

Rerun-Tryjobs: true
Bug: b/390473370
Change-Id: Iae0b48a0191fe9e4ef6db3f1e89a0c7e08466d3a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1174659
Reviewed-by: Nicolette Prevost <nicolettep@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>