image: use paletted image.
This simplifies code and makes it faster. It also produces smaller PNGs. Before: captcha.BenchmarkNewImage 500 6198230 ns/op captcha.BenchmarkImageWriteTo 100 17517110 ns/op 0.14 MB/s After: captcha.BenchmarkNewImage 500 3869968 ns/op captcha.BenchmarkImageWriteTo 200 7237604 ns/op 0.23 MB/s (Core 2 Duo @ 2.26 GHz, MacBook) Currently the background is white, not transparent, because PNG encoder doesn't support paletted images with alpha channel. Submitted CL: http://codereview.appspot.com/4432078 Change alpha to 0x00 once it's accepted to make captcha transparent.
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