### 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<String> components = String(p_name).split("/", true, 2);
	
	Vector<String> coords_split = components[0].split(":");
	if (coords_split.size() == 2 && coords_split[0].is_valid_int() && coords_split[1].is_valid_int()) {
		Vector2i coords = Vector2i(coords_split[0].to_int(), coords_split[1].to_int());
		if (tiles.has(coords)) {
			if (components.size() >= 2) {
				
				if (components[1] == "size_in_atlas") {
					r_ret = tiles[coords].size_in_atlas;
					return true;
				} else if (components[1] == "next_alternative_id") {
					r_ret = tiles[coords].next_alternative_id;
					return true;
				} else if (components[1] == "animation_columns") {
					r_ret = get_tile_animation_columns(coords);
					return true;
				} else if (components[1] == "animation_separation") {
					r_ret = get_tile_animation_separation(coords);
					return true;
				} else if (components[1] == "animation_speed") {
					r_ret = get_tile_animation_speed(coords);
					return true;
				} else if (components[1] == "animation_mode") {
					r_ret = get_tile_animation_mode(coords);
					return true;
				} else if (components[1] == "animation_frames_count") {
					r_ret = get_tile_animation_frames_count(coords);
					return true;