Tag checkerboard bitmaps as sRGB

Significantly reduces the diff between legacy and sRGB/F16 on about 25
GMs. This is just the biggest piece of low-hanging fruit. Many GMs create
N32 raster surfaces to procedurally generate source textures, and I'd like
to fix all of them. It's much easier to reason about the GMs (is sRGB
doing the right thing) when everything is tagged like this - the only
expected differences are due to filtering and blending.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2368933003

Committed: https://skia.googlesource.com/skia/+/fe843cea499ba163d53281425af210b1887d28e7
Review-Url: https://codereview.chromium.org/2368933003

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