### 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)

if (components[1] == "collision_layer") {
			r_ret = get_physics_layer_collision_layer(index);
			return true;
		} else if (components[1] == "collision_mask") {
			r_ret = get_physics_layer_collision_mask(index);
			return true;
		} else if (components[1] == "physics_material") {
			r_ret = get_physics_layer_physics_material(index);
			return true;
		}
	} else if (components.size() >= 2 && components[0].begins_with("terrain_set_") && components[0].trim_prefix("terrain_set_").is_valid_int()) {
		
		int terrain_set_index = components[0].trim_prefix("terrain_set_").to_int();
		if (terrain_set_index < 0 || terrain_set_index >= terrain_sets.size()) {
			return false;
		}
		if (components[1] == "mode") {
			r_ret = get_terrain_set_mode(terrain_set_index);
			return true;
		} else if (components.size() >= 3 && components[1].begins_with("terrain_") && components[1].trim_prefix("terrain_").is_valid_int()) {
			int terrain_index = components[1].trim_prefix("terrain_").to_int();
			if (terrain_index < 0 || terrain_index >= terrain_sets[terrain_set_index].terrains.size()) {
				return false;
			}
			if (components[2] == "name") {
				r_ret = get_terrain_name(terrain_set_index, terrain_index);
				return true;
			} else if (components[2] == "color") {
				r_ret = get_terrain_color(terrain_set_index, terrain_index);
				return true;