[3.x] Add support for saving multiple Images in BakedLightmap

[JFonS]

Instead of fitting all atlas slices into a single image, which meant there was a hard limit on the size, BakedLightmap will now save as many images as needed to fit all the slices generated by the lightmapper.

I only did some quick testing on a single scene, so it would be good to get this tested on more scenarios before merging.

The size limit is much less of an issue in master since the maximum size for images is much larger there. Still, once we see everything is working fine in 3.x, this can be forward-ported to master.

Closes #51741, implements godot-proposals#2147.

[Calinou]

I tried this on Reflection, but the resulting bake looks broken:

When atlas generation is disabled, it looks correct:

Testing project: test_lightmap_large_3.x.zip
Baking lightmaps can take a while on this scene, so I recommend setting bake quality to Low, setting the number of bounces to 1 and disabling the denoiser.

[JFonS]

I force-pushed a fix for @Calinou's issue, everything should be working now.

[akien-mga]

Could you PR that fix upstream too?

[akien-mga]

Worth noting that stb_rect_pack 1.01 would make this the default behavior (we have 1.00): https://github.com/nothings/stb/blob/af1a5bc352164740c1cc1354942b1c6b72eacb8a/stb_rect_pack.h#L44

[JFonS]

@akien-mga stb_rect_pack 1.01 doesn't have the warning issue because it gets rid of the dual modes.

Since updating to 1.01 fixes both changes I had made in this PR, I just removed them and we can do the upgrade in a separate PR.

[akien-mga]

Thanks!

[azur-wolve]

Looks awesome!
There are any advantages a part from reducing memory usage? (if understood correctly)

[Calinou]

Looks awesome! There are any advantages a part from reducing memory usage? (if understood correctly)

Supporting multiple atlases makes it possible to successfully bake lightmaps in large levels without exceeding the maximum texture size. Before multiple atlases were supported, you had to disable atlases to successfully bake lightmaps in a large scene. This was less efficient to render and resulted in a lot of files being created when baking lightmaps.

[gdlq]

Doesn't work in 3.5 RC2.
Same error as before Failed determining lightmap size. Maximum lightmap size too small?
I tried the test scene from https://github.com/godotengine/godot/files/8064047/test_lightmap_large_3.x.zip
Added 10 additional cubes from blender to the scene and changed import settings in the Lightmap Texel Size to 0.05, I should have several exr textures but I got error Maximum lightmap size too small, if I change Lightmap Texel Size to 1.0 then it bakes all in single exr texture.

[Calinou]

@gdlq Please open a new issue with a minimal reproduction project attached.