fix pngquant on Debian Jessie armhf

The first patch fixes some pointer arithmetic issues. I suspect on other platforms the size of this struct % 16 is 0 or 1 so the math works out the same. On my system it was 20 % 16 = 4 so the values would come out wrong and this assertion would fail.

The second patch fixes an issue where 1-bit grayscale images weren't being converted to RGBA before being processed as RGBA, causing them to come out as colored vertical bars. I'm not sure why this would be different on armhf, so maybe it's an issue on all Debian Jessie builds or all builds against certain libpng versions.

![x-fs8](https://f.cloud.github.com/assets/17961/1813934/2ca5675c-6eda-11e3-904b-e5041d1f74a9.png)
![x-cb](https://f.cloud.github.com/assets/17961/1813935/32a7d9d2-6eda-11e3-905e-eddd6c7594c1.png)