Implement HD textures conversion

[tonihele]

Implement HD textures conversion. This would work with a command line arg, such as -hd. The command line arg would add a new asset location to primarily use HD assets. And the asset conversion process needs a new task, AI upscaling of images. This would also be a conditional step and normally wouldn't be done.

So what we need is:

• Support conditional conversion tasks
• Implement AI driven image upscaling (use one of the trained models from the Internet)

[Trass3r]

Does it really make sense to bake this into the app?
A separate script may be enough, though there are many different tools. Maybe a section in the Readme or wiki is more appropriate.

[tonihele]

Maybe you are right. I just found this easy solution to do it. Although the trained models would increase the package/distribution size quite a bit.... At least first we should fix #343.