draw: RGBA dst fast path for scaling.

benchmark                     old ns/op      new ns/op      delta
BenchmarkScaleLargeDownNN     6124873        3348203        -45.33%
BenchmarkScaleLargeDownAB     15608417       12626534       -19.10%
BenchmarkScaleLargeDownBL     1503354937     1482605150     -1.38%
BenchmarkScaleLargeDownCR     2987623786     2937846270     -1.67%
BenchmarkScaleDownNN          1793478        935896         -47.82%
BenchmarkScaleDownAB          4277596        3405613        -20.38%
BenchmarkScaleDownBL          29932226       29268085       -2.22%
BenchmarkScaleDownCR          57563042       57322266       -0.42%
BenchmarkScaleUpNN            89694138       46216098       -48.47%
BenchmarkScaleUpAB            212318283      169267373      -20.28%
BenchmarkScaleUpBL            120899444      80215032       -33.65%
BenchmarkScaleUpCR            181116518      140140247      -22.62%
BenchmarkScaleSrcNRGBA        13229017       10620746       -19.72%
BenchmarkScaleSrcRGBA         12993292       10155919       -21.84%
BenchmarkScaleSrcUniform      3964808        1146947        -71.07%
BenchmarkScaleSrcYCbCr        15871184       12779895       -19.48%

Change-Id: I7d92bd9f4c20692c5a52ea31019fe3852e657535
Reviewed-on: https://go-review.googlesource.com/6230
Reviewed-by: Rob Pike <r@golang.org>
2 files changed