Various fixes to UV2 unwrapping and the GPU lightmapper. Listed here for
context in case of git blame/bisect:
* Fix UV2 unwrapping on import, also cleaned up the unwrap cache code.
* Fix saving of RGBA images in EXR format.
* Fixes to the GPU lightmapper:
- Added padding between atlas elements, avoids bleeding.
- Remove old SDF generation code.
- Fix baked attenuation for Omni/Spot lights.
- Fix baking of material properties onto UV2 (wireframe was
wrongly used before).
- Disable statically baked lights for objects that have a
lightmap texture to avoid applying the same light twice.
- Fix lightmap pairing in RendererSceneCull.
- Fix UV2 array generated from `RenderingServer::mesh_surface_get_arrays()`.
- Port autoexposure fix for OIDN from 3.x.
- Save debug textures as EXR when using floating point format.
