[GIFs2PNG] Allow to skip conversions that lead to semi-transparent results.

[0lhi]

In order to allow for a lossless preservation of Episodes, an option to skip the conversion of GIF-Pairs that would give a semi-transparent result is needed, as the result is inconsistent for different rendering algorithms (LogicOP, Min/Max, GIFs2RGBA).

[Wohlstand]

Implemented the thing just now!
The -n argument, and the checkbox at the Maintainer had been added!