Skia: Add skia::DrawGainmapImage function

Add the helper function skia::DrawGainmapImage, which draws a gainmap
image the same way an ordinary image would be drawn by
SkCanvas::drawImage.

This requires extra helper functions to deal with tiling the two
input textures.

Add the helper class skia::Tiling, which can be used to divide a
destination rectangle for a drawRect call into several sub-rectangles.
This is for use with SkShaders that sample from multiple SkImages,
some of which might be too large to fit into a texture. Add tests for
this functionality.

Make skia::DrawGainmapImageRect use this generic functionality.

Bug: 328665503
Change-Id: I27442d1b0df871f88795687b155f30d9f862f930
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5401806
Commit-Queue: ccameron chromium <ccameron@chromium.org>
Reviewed-by: Brian Osman <brianosman@google.com>
Cr-Commit-Position: refs/heads/main@{#1281537}