Fix -O0 build

btb reports that clang fails to link
<77ec36b9c3 (commitcomment-9992820)>.
When building at -O0, static member glow_num has its address referenced
because the compiler does not inline non-static method
glow_num_stub::operator=.  Add a definition of this symbol.

Fixes: 77ec36b9c3 ("Convert g3_draw_morphing_model to class")

similar/3d/interp.cpp

@@ -493,6 +493,7 @@ public:
 	}
 };
 
+constexpr glow_num_stub g3_draw_morphing_model_state::glow_num;
 constexpr const glow_values_t *g3_draw_morphing_model_state::glow_values;
 
 }