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")
cb83156132