Fix solid color thumb quad under large scale

If a layer has a large scale, visible_layer_rect() can't reliably
clip a quad before scaling because a "pixel" in the layer is very
large and scale-after-clip will create a quad exceeding the clip.

clip
  large-scale
    layer

Now share more code with the non-solid-color-thumb code path.

For a AppendQuads method, to ensure the clip rect is applied, it's
better to call PopulateScaledSharedQuadState() instead of creating a
shared quad state by itself.

Bug: 377948403
Change-Id: I8d7dd08fe9fc7907685bfbaa1a5fd5682688aed0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6014683
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1382613}