composite_rgba8 removed the white matte from 8-bit RGB composites with
one integer division per colour sample (3 per pixel, ~108 million on a
36 MP file). The result depends only on the sample and its alpha, so the
same expression, now unmatte_u8, fills a 256 x 256 table once per call
and each sample is a lookup. Built per call, not as a static, so no
binary (wasm included) grows by the table.
The new test runs all 65 536 (sample, alpha) pairs through
composite_rgba8 against the replaced loop, kept verbatim as the
reference.
xsvm
f2ae78ed93