Cleanups related to SkFixed.

Cleanups related to SkFixed.

Remove SK_FixedNaN. It does not seem to be supported or used anywhere in Skia, Chromium, Android, or Google3, (except accidentally by TwoPtRadial::kDontDrawT). I think supporting it would erase any benefit of SkFixed over float.

Remove SkBitmapFilter::lookup. It does not appear to be used anywhere in Skia, Chromium, Android, or Google3.

Fix a bug in SkPaint::breakText that limited it to ~5kB of text. Now it can handle more than 1GB.

BUG=skia:4632
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Review URL: https://codereview.chromium.org/1683743005