Allow Selecting ORM images and auto-split and wire them up to channels

[thyongamer]

Pre-Flight checklist

• [x] Did you check to see if this issue already exists?
• [x] This is a single feature request. (Do not put multiple feature requests in one issue)

Describe The Feature Request Below

When you import ORM images, they are not supported but auto-mapping R -> Occulsion, G -> Roughness and B -> Metallic. You currently have to setup this in the default material shader to get it to work, or use a pixel editor to split the files into 3 files. Many texture libraries use ORM (even GLTF) and supporting this with a "mapping drop-down" in each channel will be fantastic, e.g.

Load the ORM image into Metallic. Then have Color Mapping drop down [ B Channel ] Load the ORM image into Roughness. Then have Color Mapping drop down [ G Channel ]

so you can even then support the ORM baking you use.

[ucupumar]

Currently ucupaint can't use a specific image channel as a layer channel input yet. This will be possible only when #128 is implemented.