### 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)

ERR_FAIL_INDEX(p_from_index, occluders.size());
	ERR_FAIL_INDEX(p_to_pos, occluders.size() + 1);
	occluders.insert(p_to_pos, occluders[p_from_index]);
	occluders.remove_at(p_to_pos < p_from_index ? p_from_index + 1 : p_from_index);
}
void TileData::remove_occlusion_layer(int p_index) {
	ERR_FAIL_INDEX(p_index, occluders.size());
	occluders.remove_at(p_index);
}
void TileData::add_physics_layer(int p_to_pos) {
	if (p_to_pos < 0) {
		p_to_pos = physics.size();
	}
	ERR_FAIL_INDEX(p_to_pos, physics.size() + 1);
	physics.insert(p_to_pos, PhysicsLayerTileData());
}
void TileData::move_physics_layer(int p_from_index, int p_to_pos) {
	ERR_FAIL_INDEX(p_from_index, physics.size());
	ERR_FAIL_INDEX(p_to_pos, physics.size() + 1);
	physics.insert(p_to_pos, physics[p_from_index]);
	physics.remove_at(p_to_pos < p_from_index ? p_from_index + 1 : p_from_index);
}
void TileData::remove_physics_layer(int p_index) {
	ERR_FAIL_INDEX(p_index, physics.size());
	physics.remove_at(p_index);
}
void TileData::add_terrain_set(int p_to_pos) {