Fully implement the 'Merge Layers' operator.

[LoganFairbairn]

No description provided.

[LoganFairbairn]

In some cases, such as when two layers both use images to display data in their channels (as opposed to using procedural maps or data from values) it should be possible to merge the layers which can be extremely useful in some cases, and can help users optimize their materials as they work.

Layers might need to be marked as 'Paint Layers' for them to be able to be properly identify-able as being able to be merged.

[LoganFairbairn]

This could be really good for helping users reduce the number of layers they use, however the implementation of it will likely be pretty difficult. Making an optimized, fast operator for merging full material layers would likely take a lot of time, so I'm back-logging this ticket for the time being.

[LoganFairbairn]

I've added an experimental merge layers operator as a new experimental feature, but making it efficient when the material uses many channels might not be possible.

Merging layers also requires masks to be converted to pixel data too, and when pixel data is calculated, it needs to take into account layer projection to properly merge.

[LoganFairbairn]

After playing around with some numpy magic to attempt and merge layers faster than a baking operation would, I determined it's not feasible due to needing to account for 3D projection methods, and nodes that are difficult to translate to pixel data.

The merge layers operator however, can still be extremely useful in some cases if for nothing else than helping users manage their texture memory usage and layer count. I'm going to implement this feature using a modal baking operator, similar to how the mesh map and texture exporting operators in this add-on already work.