[Feature] Allow converting PNG to CZ without providing source

[masagrator]

Since we don't have CZ2, CZ4 and CZ5 importer, current image import is blocking PNG images from converting into any CZ textures.

I know that some informations from header after converting to PNG are lost.
So either allow in CMD to set custom values, or just put default values (if image is 1280x720, put anchor at (0,0) and other dimensions at 1280x720) and people will fix it with hex editor if necessary.