Inform ProXL that Gc is no longer being "held onto" by the programmer. As soon as no drawables refer to it, Gc will be destroyed.

Warning: A gc is considered to be "held onto" when it is created, or when it is got from a drawable (e.g. get_window_attributes). It is the responsibility of the programmer to ensure that she is not releasing a gc that is being held by another branch of her code. For example, If a gc is created, asserted somewhere, and put into a drawable, and later is accessed through the drawable, the programmer must not release it at this point, since it is still in the database and may be used. Be careful.