What did we learn today?  255 != 256

vcvt_n_f32_u32 and _u32_f32 work in power-of-2 fixed point, so (...,8)
meant 'please multiply or divide by 256'.  We need to use 255.  :(

BUG=skia:

Review URL: https://codereview.chromium.org/1204363002
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