Move switching of screen's current bitmap to after layout updates

[enikland3]

Currently video_manager::frame_update() updates the screen object by calling finish_screen_updates(), that calls screen.update_quads(), where the current m_curbitmap is set as the bitmap to be used by the frame texture. The code then switches the m_curbitmap pointer to the other bitmap available to blit the next frame. The execution returns to after the last call was made in video_manager::frame_update() and continues through the code. One of the remaining calls is machine().osd().update(), where the layout is rendered. If the layout contains a script that calls screen:pixels() to obtain the data of pixels in the frame, it won't get the data of the last frame. To make this work, these changes move switching of screen's current bitmap to after the layout is updated.