most unused arg warnings corrected.

- removed deprecated bitmap arg from IMB_allocImBuf (plugins will need updating).
- mostly tagged UNUSED() since some of these functions look like they may need to have the arguments used later.

source/blender/editors/transform/transform_constraints.c

@@ -593,7 +593,7 @@ void setUserConstraint(TransInfo *t, short orientation, int mode, const char fte
 
 /*----------------- DRAWING CONSTRAINTS -------------------*/
 
-void drawConstraint(const struct bContext *C, TransInfo *t)
+void drawConstraint(TransInfo *t)
 {
 	TransCon *tc = &(t->con);