image/color, image/draw: speed up color conversions with bit twiddling

[josharian]

CL 21910 sped up image/color BenchmarkYCbCrToRGB and image/draw BenchmarkYCbCr considerably by removing branches. Investigate doing this for other, similar conversions.

cc @martisch @nigeltao

[gopherbot]

CL https://golang.org/cl/22411 mentions this issue.

[gopherbot]

CL https://golang.org/cl/31773 mentions this issue.