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/render/intern/source/volume_precache.c

@@ -89,7 +89,7 @@ int intersect_outside_volume(RayObject *tree, Isect *isect, float *offset, int l
 }
 
 /* Uses ray tracing to check if a point is inside or outside an ObjectInstanceRen */
-int point_inside_obi(RayObject *tree, ObjectInstanceRen *obi, float *co)
+int point_inside_obi(RayObject *tree, ObjectInstanceRen *UNUSED(obi), float *co)
 {
 	Isect isect;
 	float vec[3] = {0.0f,0.0f,1.0f};