### 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)

Vector<int> source_to_remove;
	for (const KeyValue<int, int> &E : source_level_proxies) {
		if (has_source(E.key)) {
			source_to_remove.push_back(E.key);
		}
	}
	for (int i = 0; i < source_to_remove.size(); i++) {
		remove_source_level_tile_proxy(source_to_remove[i]);
	}
	
	Vector<Array> coords_to_remove;
	for (const KeyValue<Array, Array> &E : coords_level_proxies) {
		Array a = E.key;
		if (has_source(a[0]) && get_source(a[0])->has_tile(a[1])) {
			coords_to_remove.push_back(a);
		}
	}
	for (int i = 0; i < coords_to_remove.size(); i++) {
		Array a = coords_to_remove[i];
		remove_coords_level_tile_proxy(a[0], a[1]);
	}
	
	Vector<Array> alternative_to_remove;
	for (const KeyValue<Array, Array> &E : alternative_level_proxies) {
		Array a = E.key;