fix some warnings (#660)

examples/component/button.cpp

examples/component/button_animated.cpp

examples/component/button_in_frame.cpp

examples/component/button_style.cpp

examples/component/canvas_animated.cpp

examples/component/checkbox.cpp

examples/component/checkbox_in_frame.cpp

examples/component/collapsible.cpp

examples/component/composition.cpp

examples/component/custom_loop.cpp

examples/component/dropdown.cpp

examples/component/flexbox_gallery.cpp

examples/component/focus.cpp

examples/component/focus_cursor.cpp

examples/component/gallery.cpp

examples/component/homescreen.cpp

examples/component/input.cpp

examples/component/input_style.cpp

examples/component/linear_gradient_gallery.cpp

examples/component/maybe.cpp