Color STL support

[anoved]

Patch stlwriter to store color data pulled from a second image in the attribute byte count fields. Not an official standard but there seems to be support for it here and there. https://en.wikipedia.org/wiki/STL_%28file_format%29#Color_in_binary_STL

[anoved]

This is worth pursuing to support printing color terrain models, especially through i.Materialise.