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There exists code that copies an IOSurface to a CAMetalLayer,
because simply setting a CALayer's contents to an IOSurface that
has an HDR color space is not sufficient to guarantee that the
content will be displayed as HDR (it did in early versions of
10.16, but now does not).

Separately, there is now desire to have tonemapping on lower
end HDR devices. This is fairly easy to wrangle into the
pipeline in the place where we do the copy.

In this patch, replace the blit with a shader that does
the blit. Also, change the CAMetalLayer from matching the
IOSurface to being float16. Patches will follow will include
- Change the CAMetalLayer to be in extended-linear-sRGB
  color space, by adding limited color conversion to the
  shader.
- Add tonemapping in the extended-linear-sRGB color space.
- Set the tonemapping parameters based on the NSScreen
  properties.

Bug: 1101041
Change-Id: If1aaa6081730212611cb4cf58d8f90d5957dddf7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2352027
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798466}