gm: Don't run the gm binary until the first chunk is written.

Allows more graceful error handling when the gm binary is not available.

Also save some additional info on each filter instance.
Update inkscape to 0.0.4.
Update optipng to 0.0.5.
Update pngcrush to 0.0.4.
Update pngquant to 0.1.2.
Update jpegtran to 0.0.5.
0.1.15