callable(I, T) returns True when I is an object of a type T or
of a subtype of T. Also implemented a measure to avoid an
infinite loop when user-defined predicate and function classes
do not properly overload the __call__ method (including the
cases of directly instantiating the base classes such as
UnaryPredicate0D and BinaryPredicate1D).
All of the reference to the original Material class were renamed to FrsMaterial to resolve a name collision with Blender. To keep the window context necessary to draw the strokes after RE_Database_FromScene has been called, the display_clear function is used.
