Automatically setting image codec

[Migushthe2nd]

I quite often used video2x to upscale images. v6.0.0 currently does not save png images as png format by default as the libx264 vcodec is used. It would be nice if it was automatically set to the png codec (similarly e.g. jpeg2000 for jpegs)

[k4yt3x]

I didn't design this version to work with images at all actually... It's something we could optimize for in the future.

[k4yt3x]

Added in: k4yt3x/video2x-qt6@8a1b5b3. Will be in the next release.

Now it automatically sets image output codec to PNG, which is probably a good default for decent quality output and compatibility.