Silence mixed declaration/code warnings.
Silence mixed declaration/code warnings.

Also switch to simple index/count loops for rb_monitor_video_modes,
rb_monitor_set_gamma_ramp, rb_glfw_get_joystick_axes, and
rb_glfw_get_joystick_buttons.

rb_lookup_window and some callbacks made slightly safer.
rb_window_get_current_context no longer duplicates rb_lookup_window's
code.

nilium/ruby-glfw3,nilium/ruby-glfw3