[Oversight] Don't reencode VTF if no format is specified

[SirYodaJedi]

Using v0.1.1.

Currently, if running vtex2.exe convert -c 9 input.vtf, the output VTF is RGBA8888, regardless of what format the original is. It seems that it is reencoding the textures, wasting time and causing minor generation loss.

(As an aside, it would be nice if the same could be done for DDS, so we could losslessly remux between DDS and VTF.)