Some benchmark numbers below. The image/draw fast-paths show dramatic
improvement, the generic slow-paths show a smaller slow-down.

BEFORE
png.BenchmarkEncodePaletted      200       8203800 ns/op      37.45 MB/s
png.BenchmarkEncodeRGBOpaque         100      26940440 ns/op      45.61 MB/s
png.BenchmarkEncodeRGBA       20      73821000 ns/op      16.65 MB/s
jpeg.BenchmarkEncodeRGBOpaque         50      35598640 ns/op      34.52 MB/s
draw.BenchmarkFillOver	     500	   4024226 ns/op
draw.BenchmarkFillSrc	   10000	    152736 ns/op
draw.BenchmarkCopyOver	     500	   3452824 ns/op
draw.BenchmarkCopySrc	   50000	     73218 ns/op
draw.BenchmarkNRGBAOver	     500	   3941234 ns/op
draw.BenchmarkNRGBASrc	    1000	   2484400 ns/op
draw.BenchmarkYCbCr	    1000	   2609005 ns/op
draw.BenchmarkGlyphOver	    2000	   1169575 ns/op
draw.BenchmarkRGBA	     200	   9031390 ns/op
draw.BenchmarkGenericOver	      50	  34636620 ns/op
draw.BenchmarkGenericMaskOver	     100	  16561150 ns/op
draw.BenchmarkGenericSrc	     100	  13873760 ns/op
draw.BenchmarkGenericMaskSrc	     100	  25198860 ns/op

AFTER
png.BenchmarkEncodePaletted      200       8206600 ns/op      37.43 MB/s
png.BenchmarkEncodeRGBOpaque         100      26129530 ns/op      47.03 MB/s
png.BenchmarkEncodeRGBA       20      75776750 ns/op      16.22 MB/s
jpeg.BenchmarkEncodeRGBOpaque         50      37192940 ns/op      33.04 MB/s
draw.BenchmarkFillOver	     500	   3008134 ns/op
draw.BenchmarkFillSrc	   10000	    154214 ns/op
draw.BenchmarkCopyOver	    1000	   2169988 ns/op
draw.BenchmarkCopySrc	   50000	     73095 ns/op
draw.BenchmarkNRGBAOver	    1000	   2491079 ns/op
draw.BenchmarkNRGBASrc	    2000	   1361244 ns/op
draw.BenchmarkYCbCr	    1000	   2554269 ns/op
draw.BenchmarkGlyphOver	    2000	   1042225 ns/op
draw.BenchmarkRGBA	     100	  10233340 ns/op
draw.BenchmarkGenericOver	      50	  38421560 ns/op
draw.BenchmarkGenericMaskOver	     100	  17521190 ns/op
draw.BenchmarkGenericSrc	     100	  16351200 ns/op
draw.BenchmarkGenericMaskSrc	     100	  26538190 ns/op

R=r
CC=golang-dev
https://golang.org/cl/4675076