jpeg: speed up RGBA encoding ~%50
Avoids image.At(), color.RGBA(), opposing 8 bit shifts,
and min function calls in a loop. Not as pretty as before,
but the pure version is still there to revert back to
later if/when the compiler gets better.
before (best of 5)
jpeg.BenchmarkEncodeRGBOpaque 50
64781360 ns/op 18.97 MB/s
after (best of 5)
jpeg.BenchmarkEncodeRGBOpaque 50
42044300 ns/op 29.23 MB/s
(benchmarked on an HP z600; 16 core Xeon E5520 @ 2.27Ghz)
R=r, r2, nigeltao
CC=golang-dev
https://golang.org/cl/
4433088