Assorted fixes to UV unwrapping and GPU lightmapper

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.

modules/tinyexr/image_saver_tinyexr.cpp

@@ -169,7 +169,7 @@ Error save_exr(const String &p_path, const Ref<Image> &p_img, bool p_grayscale)
 		{ 0 }, // R
 		{ 1, 0 }, // GR
 		{ 2, 1, 0 }, // BGR
-		{ 2, 1, 0, 3 } // BGRA
+		{ 3, 2, 1, 0 } // ABGR
 	};
 
 	int channel_count = get_channel_count(format);