Add a debug draw density property to BakedLightmap

[Calinou]

When enabled, debug_draw_density will draw a checkerboard on the baked lightmap. This is done by modifying the lightmap data itself.

This can be used to diagnose issues related to lightmap texel density and lightmap seams.

Ideally, this would be done directly in the shader to avoid having to bake lightmaps again. I don't know how to implement that though.
Edit: This is likely done by passing the lightmap texture size uniform is debug density is enabled, even if bicubic sampling is disabled in the project settings. However, the issue is that density debugging is configured on a per-node basis rather than a global project setting (like bicubic sampling).

This partially addresses godotengine/godot-proposals#3213.

Preview

debug_draw_density disabled	debug_draw_density enabled

[s-ilent]

This is a useful feature, but the implementation has some issues.

With this, previewing the lightmap resolution becomes a destructive operation. There is no way to revert it short of rebaking the lightmap. That limits the use of this feature. Being able to preview changes to the lightmap density will then require rebaking the entire lightmap to view the changes against a clean canvas, adding time to iteration.

As a candidate for merging, I think this is a bad proposal. As a stopgap solution, it's okay. But a non-destructive shader based approach is really a better solution with less footguns.

[jcostello]

@s-ilent I completely agree. Its OK for debugin issues before baking as right now we don't have any tool. But as you said, its a destructive method and in the future it will be nice to have it implemented in the shaders.
@Calinou in master there is a debug draw that display the lighting information. Could that be use to apply a texture over it?

[jcostello]

Also, this should work with the lightmap scale property in the object

[Calinou]

Superseded by #62987, which is a better implementation that doesn't require rebaking lightmaps.