Fixed: Erroneous attempt to free memory in SB_DestroySurface()

Memory for the BiasSurfaces and vertexillum_ts is block-allocated by
SB_InitForMap(), therefore it can not be free'd piece-wise with the
existing mechanism used by SB_DestroySurface().

doomsday/client/src/render/rend_bias.cpp

@@ -156,8 +156,9 @@ void SB_DestroySurface(BiasSurface *bsuf)
         }
     }
 
-    Z_Free(bsuf->illum);
-    Z_Free(bsuf);
+    /// Bias surfaces and vertex illum data is block-allocated.
+    //Z_Free(bsuf->illum);
+    //Z_Free(bsuf);
 }
 
 int SB_NewSourceAt(coord_t x, coord_t y, coord_t z, float size, float minLight,