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.