Update tex2ngl.py to work for more types of images

[miracade]

I recently found that tex2ngl currently doesn't work for images which aren't in RGB or RGBA format (for example, PNGs in indexed mode). PIL's Image.convert() method is an easy fix

[miracade]

Make sense to me. From my testing, converting from a format with an alpha channel to RGB doesn't raise any warnings or anything so this shouldn't break anything