feat(ui): canvas entity merging

[psychedelicious]

Summary

• Refactor CanvasCompositorModule to be more flexible. The core functionality should be identical, but it now supports more types of compositing, as required by the new merge functions. Technically, it supports Merge Selected, but that is predicated on supporting multi-select on Canvas, which is out of scope for this PR.
• Change Merge Visible behaviour to match raster editors. It creates a new layer, does not delete anything.
• Add Merge Visible support for Regional Guidance. The masks are merged into a new RG layer with default RG settings.
• Add Merge Visible support for Control Layers. The layers are merged using the "lighter" blend mode, which stacks up the lightness of every pixel. This leaves black pixels black.
• Add Merge Down support for all layers. This works the same as in Affinity. The merged layers are discarded and an new layer created. Unlike Merge Visible, Merge Down merges disabled layers. RG and CL layers lose their settings. CL layers merge with the same blending as Merge Visible.
• Added Save to Assets to the Inpaint Mask and Regional Guidance ctx menus. Not sure why this wasn't there before but as of this PR, all layer types have the same layer manipulation capabilities, so it makes sense to have the menu item.

Related Issues / Discussions

offline discussion

QA Instructions

• Try Merge Down for each layer type
• Try Merge Visible for Control Layers and Regional Guidance
• Do some inpainting and outpainting (testing the core compositor module functionality)

There's one major question mark - the way Control Layers get merged.

• Does the "lighter" blend mode do what we want? We can merge the layers according to any "composite operation", OR use a custom algorithm. For example, we could use the "transparency effect" algo before compositing, in which case two canny layers would merge to a transparent layer with white lines.
• Do we want an option to merge them exactly the same as Raster Layers (stack them up without any blending)?

Merge Plan

n/a

Checklist

• The PR has a short but descriptive title, suitable for a changelog

[hipsterusername]

For the time being, lighter most likely serves the majority use case for merging. I think other layer merging can be explored as feedback rolls in

[hipsterusername]

Approving but still need to test.

[psychedelicious]

Made some improvements the canvas' internal caching logic, as used by the compositor module. These changes improve UI performance and result in fewer images being created on the server:

• Cache image data from composite layers (i.e. the actual pixel data used for generation mode calculation)
• Cache transparency calculation results (i.e. the result of calculating the transparency of the image data for generation mode calculation)
• Do not invalidate these caches when non-renderable state changes. For example, when an RG's prompt changes, we shouldn't invalidate the caches for its rendered mask, because the a change to the prompt never changes the mask.