Run optipng before pngcrush
Run optipng before pngcrush

Seems to avoid optipng choking on pngcrush's output with this error:

Warning: Too few colors in PLTE
Reducing image to 8 bits/pixel, 256 colors (253 transparent) in palette
Recoverable errors found in input. Rerun OptiPNG with -fix enabled.
Error: Previous error(s) not fixed

assetgraph/assetgraph-builder