Add TGA support

[NiveusLuna]

I added TGA support. I tried to follow your conventions and pull request requirements.

I did move the "flatten" command to only run if padding is enabled. I don't know if this will cause issues; I did not test anything on other formats. I did this because flattening destroys transparency in TGAs. I don't know if this will cause issues; I did not test anything on other formats.