### 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)

Ref<Texture2D> txt = get_texture();
	if (!txt.is_valid()) {
		return Vector2i();
	}
	ERR_FAIL_COND_V(texture_region_size.x <= 0 || texture_region_size.y <= 0, Vector2i());
	Size2i valid_area = txt->get_size() - margins;
	
	Size2i grid_size;
	if (valid_area.x >= texture_region_size.x && valid_area.y >= texture_region_size.y) {
		valid_area -= texture_region_size;
		grid_size = Size2i(1, 1) + valid_area / (texture_region_size + separation);
	}
	return grid_size;
}
bool TileSetAtlasSource::_set(const StringName &p_name, const Variant &p_value) {
	Vector<String> components = String(p_name).split("/", true, 2);
	
	Vector<String> coords_split = components[0].split(":");
	Vector2i coords = TileSetSource::INVALID_ATLAS_COORDS;
	if (coords_split.size() == 2 && coords_split[0].is_valid_int() && coords_split[1].is_valid_int()) {
		coords = Vector2i(coords_split[0].to_int(), coords_split[1].to_int());
	}
	
	if (coords != TileSetSource::INVALID_ATLAS_COORDS) {
		
		if (!has_tile(coords)) {
			create_tile(coords);
		}
		if (components.size() >= 2) {