Add layer mask duplication operator

[BittieByte]

Initial version of the layer mask duplication operator. The feature is currently functional but incomplete.

Known issues:

• Duplicate creates a new decal object instead of reusing the original source.
• Duplicated mask is not inserted directly below the original.
• Some properties such as transform values, intensity, and modifiers are not yet copied.

CLOSES #153

[ucupumar]

Oh yeah, I don't realize that you can just create a new mask with all properties copied from the original mask. I thought duplicating a mask is similar to duplicating a layer, which actually copies the node manually. Okay, I'll test this later. Thanks for the PR! 👍

[ucupumar]

I made some comments on the implementation, maybe you can address those. Nice work otherwise! 👍

[ucupumar]

Copying the intensity_value property directly won't use the updated value that the original mask actually uses. The property only stores the value only when the mask is disabled. This is because ucupaint prioritizes the value that actually exists as node input in the material rather than the one on the python property. Ucupaint always deletes unnecessary nodes and inputs when no entity is using them, so it's for optimization purposes.

You can use get_entity_prop_value and set_entity_prop_value functions to get and set the entity property. Those will guarantee use the correct value. You learn what those functions do in common.py

I hope that's not too confusing, please ask me if you have any questions.

[ucupumar]

I think there's should be an option to duplicate packed images, like on Duplicate Layer operator

[ucupumar]

For transformation, it's a bit different to copy the values, you need to get the mapping node from the original mask and copy the values to the new mapping node

[ucupumar]

This one also should use get_entity_prop_value and set_entity_prop_value