Outline Looks Different From Preview

[KittyMcTophat]

Pixelorama version:
0.10.2-stable

OS/device including version:
Windows 10
NVIDIA GeForce RTX 3050 Ti Laptop GPU
GeForce Game Ready Driver Version 516.59

Issue description:
When applying an outline to an image with multiple layers, it doesn't look the same as in the preview

Steps to reproduce:
Make a new project
Add another layer
Draw two overlapping shapes on both layers
Apply an outline

I've attached screenshots of what it looks like (pay attention to the parts of the rectangles that overlap)

[OverloadedOrama]

In this case, the generated result is correct, since the right-most rectangle should be rendered on top of the other, due to them being in different layers. The preview currently does not take layers into account; rather it blends all of them together in a single image and applied the effect there. To fix this, we'd need to somehow apply the effect to every layer and then blend them all together, which I'm not sure how trivial it is to implement.

Another idea would be to get rid of the preview image completely and instead show the preview directly on the canvas, which is what most other art software do, but that would also need a lot of changes on how we currently render layers.