xcf: format v12 support

This is a patch over MR !108 by @sandsmark. Martin has done a great job implementing support and I find it a shame not to use it.

I made sure that the results are the same as the current version and fixed the problems of pixels with wrong colors with color depth grater than 8 bits. I also fixed conversion errors on mask and gray images (16/32 bits).
Unfortunately the internal rendering engine of the original code is 8-bit so I always forced the output of 8-bit images to correct the problems (see image below). Since it is a plugin with a "rendering engine", the tests to seriously validate it are potentially endless (as the original version is not perfect).

Errors of the original version of the MR (right) which should no longer occur:
![image](/uploads/9ef24eb5436bd19ff1fb428242a9c119/image.png)