Fix exploding ring shadows in PWR Retro Maze

(note that the software renderer skips shadows with negative scale too, just
another example of anomalies caused by duplicating the entire backend)

src/hardware/hw_main.c

@@ -3157,6 +3157,10 @@ static void HWR_DrawDropShadow(mobj_t *thing, fixed_t scale)
 
 	R_InterpolateMobjState(thing, R_GetTimeFrac(RTF_LEVEL), &interp);
 
+	// yes, this can happen. but it only creates exploding shadows in SOME maps. like MAPH9
+	if (interp.radius <= 0)
+		return;
+
 	// sprite offset
 	interp.x += thing->sprxoff;
 	interp.y += thing->spryoff;