Convert manual push_style_color / pop_style_color in _gui_func to use the
imscope context manager so the pop is exception-safe via Python's with
statement. Manual push/pop can desync if render_main_interface raises
mid-render, causing 'PopStyleColor() too many times!' imGui assertion
on subsequent frames.
The try/except around render_main_interface was already there but the
pop was outside it, so the pop count could exceed the push count when
an exception short-circuited the render.
