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To draw.Draw a 32x32 image.Alpha 10000 times,
Before: 633ms with 10000 mallocs
After: 49ms with 0 mallocs

These times are just blitting an image.Alpha, and do not include
rasterizing a glyph's vector contours to an image.Alpha.

The "generic" test case in draw_test.go tests this fast path.

R=rsc
CC=golang-dev
https://golang.org/cl/1122043