Fix V3 compatibility for LayeredDiffusionDecodeRGBA node

[Geeknasty]

Updates LayeredDiffusionDecodeRGBA in layered_diffusion.py for ComfyUI V3 schema changes (from PR #10174).
Fixes: #131

Issue

Original code broke with error:

!!! Exception during processing !!! 'JoinImageWithAlpha' object has no attribute 'join_image_with_alpha'
[Traceback omitted for brevity]

Fix

- return JoinImageWithAlpha().join_image_with_alpha(image, alpha)
+ return JoinImageWithAlpha.execute(image, alpha)

Tested and functional.

[mjaniszew]

I've somehow didn't noticed this PR before opening my own: #135

Feel free to close mine as duplicate but before doing that please take a look on my changes, and question there. My PR slightly differs and also contains backwards compatibility check. I'm not sure whether it's required after all since I don't have too much experience with comfyui nodes, but I've found it useful for my case to have simple check for V1/V3