SPB special stage experiment

If you smuggle an SPB to after the death of the UFO Catcher, spawn a single, free-on-straightways manta ring before it blows up.
Testing shows that you can get hit by the SPB's explosion only if you use the free manta ring just before a sneaker panel... but also Sal thinks it's cool and I think it adds depth

src/objects/spb.c

@@ -865,6 +865,13 @@ void Obj_SPBThink(mobj_t *spb)
 
 			if ((bestMobj = K_GetPossibleSpecialTarget()) == NULL)
 			{
+				// experimental - I think it's interesting IMO
+				Obj_MantaRingCreate(
+					spb,
+					spb_owner(spb),
+					NULL
+				);
+
 				spb->fuse = TICRATE/3;
 				spb_nothink(spb) = spb->fuse + 2;
 			}