Passing unrecognized formats to gdImageGd2 results in corrupted files

The code to write GD2 images checks for unrecognized `fmt`s and [adjusts them to reasonable values](https://github.com/libgd/libgd/blob/gd-2.2.3/src/gd_gd2.c#L866-L874). However, for truecolor images the then correct value would be further increased by 2, resulting in an incorrect format being written to the file header.
