re-enable compression for png/svg only (no gif/jpeg)

This re-commits the PNG and SVG file compression from PR #105 but leaves out all GIF and JPEG files.

This follows PR #117, which reverted these compressed files when a bug was discovered with a 1x1 transparent GIF that had its global color table removed by the [Gifsicle](http://www.lcdf.org/gifsicle/) optimization utility, which triggered a bug related to transparent image overlaying an CSS-sprite background image in current Blink-based browser versions on Windows and OSX platforms.

Considering the lossless optimizations on the GIF and JPEG files did not result in significant filesize savings (compared to the significant savings from PNG quantization), it is not much of a drawback to remove those file types from this commit for the sake of being conservative.