### Godot version

5ee983188de97ae027f9b9c1443438063f708a7e

### System information

Windows 11

### Issue description

Godot editor will throw an error and fail to generate preview icons for terrain tiles which use a base image which is not RGBA8 format.
`core\io\image.cpp:2788 - Condition "format != p_src->format" is true.`

### Steps to reproduce

- Import a non-RGBA8 image into Godot.
- Create a TileMap
- Create an associated TileSet
- Create a terrain for this tileset
- Add the image as an atlas
- Go to the paint tab
- Paint the terrain on the TileSet
- An error will occur any time terrains are edited in the TileSet editor or selected in the TileMap editor terrain tab.

### Minimal reproduction project

[tileset_terrain_bug.zip](https://github.com/godotengine/godot/files/13264213/tileset_terrain_bug.zip)

// IGNORED FOR NOW, they seem duplicated data compared to the shapes array
		} else if (what == "shape") {
		} else if (what == "shape_offset") {
		} else if (what == "shape_transform") {
		} else if (what == "shape_one_way") {
		} else if (what == "shape_one_way_margin") {
		}
		// IGNORED FOR NOW, maybe useless ?
		else if (what == "occluder_offset") {
			// Not
		} else if (what == "navigation_offset") {
		}
		*/
		} else if (what == "z_index") {
			ctd->z_index = p_value;
			
			_compatibility_conversion();
		} else {
			return false;
		}
	} else {
#endif 
		
		if (components.size() == 2 && components[0].begins_with("occlusion_layer_") && components[0].trim_prefix("occlusion_layer_").is_valid_int()) {
			
			int index = components[0].trim_prefix("occlusion_layer_").to_int();
			ERR_FAIL_COND_V(index < 0, false);
			if (components[1] == "light_mask") {
				ERR_FAIL_COND_V(p_value.get_type() != Variant::INT, false);
				while (index >= occlusion_layers.size()) {