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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...