Processes an intuition message. Normally, you should not use this
function and call GT_GetIMsg() instead. If this functions returns
with a value != NULL, you have to call GT_PostFilterIMsg(), when
you are done with processing the message. If it return a NULL
pointer, you have to ReplyMsg() the message, you passed to
GT_FilterIMsg().
INPUTS
imsg
pointer to the intuition message to process
RESULT
Either a pointer to a processed intuition message or NULL, in which
case the message had only meaning to gadtools.