### 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, TileMapCell> &E : pattern) {
		Vector2i coords = E.key;
		if (p_size.x <= coords.x || p_size.y <= coords.y) {
			ERR_FAIL_MSG(vformat("Cannot set pattern size to %s, it contains a tile at %s. Size can only be increased.", p_size, coords));
		};
	}
	size = p_size;
	emit_changed();
}
bool TileMapPattern::is_empty() const {
	return pattern.is_empty();
};
void TileMapPattern::clear() {
	size = Size2i();
	pattern.clear();
	emit_changed();
};
bool TileMapPattern::_set(const StringName &p_name, const Variant &p_value) {
	if (p_name == "tile_data") {
		if (p_value.is_array()) {
			_set_tile_data(p_value);
			return true;
		}
		return false;
	}
	return false;
}
bool TileMapPattern::_get(const StringName &p_name, Variant &r_ret) const {