Fix overflow in STL header colour reading.

When reading the STL header for a "COLOR=rgb" part, the bytes were treated as signed chars, when in fact they can range from 0-255. This meant that any value greater than 127 would overflow, leading to an incorrect colour.

This change fixes the issue by treating the header as unsigned chars.
Merge pull request #313 from ajshort/patch-1

Fix overflow in STL header colour reading.