Make 2D shadows respect z_index
#93881
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AThousandShips
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z_index
AThousandShips
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bug
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cherrypick:4.2
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This PR adds the ability to sort 2D light occluders using their z_index. Fixes #64939. Now, light occluders only cast shadows on objects that have a lower or equal z_index. I had written another PR that did the same as this one a while ago (#86689), but I didn't really like the approach I took since it was on the GPU side. It also now works on directional lights and on both forward+ and mobile renderers.
Old shadows on the left, new ones on the right:
This approach doesn't modify any shaders or low-level graphics code, and instead uses only the CPU. This is achieved by calculating the shadow map inside
RendererCanvasRenderRDinstead ofRendererViewport. The shadow map gets recalculated if the lowest z_index of all active occluders is smaller than the current object's z_index, since the occluder then has to be removed from the shadow map.Performance-wise, in the best-case scenario where only one z_layer is used for all objects, the performance is the same since the shadow map is only calculated once. In the worst-case scenario, the shadow map gets calculated as many times as there are z_layers occupied by occluders. This shouldn't be a problem because usually you only need a few z_layers for occluders.
I hope that this PR can get merged since I feel like 2D shadows are currently unusable. This is still my first time contributing, so please let me know if I need to change or add anything!