### 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)

for (const KeyValue<Vector2i, TileSetAtlasSource::TileAlternativesData> &E : tiles) {
		if (!has_room_for_tile(E.key, E.value.size_in_atlas, E.value.animation_columns, E.value.animation_separation, E.value.animation_frames_durations.size(), E.key)) {
			return true;
		}
	}
	return false;
}
Vector<Vector2i> TileSetAtlasSource::get_tiles_outside_texture() const {
	Vector<Vector2i> to_return;
	for (const KeyValue<Vector2i, TileSetAtlasSource::TileAlternativesData> &E : tiles) {
		if (!has_room_for_tile(E.key, E.value.size_in_atlas, E.value.animation_columns, E.value.animation_separation, E.value.animation_frames_durations.size(), E.key)) {
			to_return.push_back(E.key);
		}
	}
	return to_return;
}
void TileSetAtlasSource::clear_tiles_outside_texture() {
	LocalVector<Vector2i> to_remove;
	for (const KeyValue<Vector2i, TileSetAtlasSource::TileAlternativesData> &E : tiles) {
		if (!has_room_for_tile(E.key, E.value.size_in_atlas, E.value.animation_columns, E.value.animation_separation, E.value.animation_frames_durations.size(), E.key)) {
			to_remove.push_back(E.key);
		}
	}
	for (const Vector2i &v : to_remove) {
		remove_tile(v);
	}
}
PackedVector2Array TileSetAtlasSource::get_tiles_to_be_removed_on_change(Ref<Texture2D> p_texture, Vector2i p_margins, Vector2i p_separation, Vector2i p_texture_region_size) {