Improve png2src
The tool `w4 png2src` converts PNG files to the selected source-code.

This is useful. But it has some limitations:
1. It has to be a PNG file; other formats are not supported
2. It has to be Index; Images in RGB(A) format are not accepted

I suggest we change those two points.

Point 1 requires we taka a look at what other libs are out there.

Point 2 can be overcome if we simply create a color-map on the fly.


I'll look into this and try to get it done, before we reach v2.2.0