[graph] Correct function pointer types

Building the demos with the CFI sanitizer detects a number of uses of
undefined behavior in the Minimalist Graphics Subsystem where an
indirect function call is made through a pointer of a different type.
All of these cases worked in practice since the differing argument
types were the same size and would have the same pointer value at
runtime.

Change the functions to take the correct types and downcast inside the
function.

* graph/beos/grbeos.cpp: remove casts

* graph/mac/grmac.c: correct return type of `listen_event`, remove casts

* graph/os2/gros2pm.c: correct function signatures, downcast inside
functions, remove function pointer casts

* graph/win32/grwin32.c: ditto

* graph/x11/grx11.c: ditto