Relocate unit testing of gui_button && gui_listbox

* examples/gui_button.cpp: Relocate unit testing portion of code to
ButtonLayoutTest.

* examples/gui_listbox.cpp: Relocate unit testing portion of code to
ListBoxLayoutTest.

* examples/gui_button.cpp: Drop render window's resolution down to
640x480 (was 800x600) to match ButtonLayoutTest. This is officially
done simply to preserve ease of comparing screen-shots, but the
resolution drop also has two other important reasons: a) helps
performance considerably on older systems (windev); b) hopefully
helps ease porting to mobile platforms down the road.

* examples/gui_button.cpp: Add resizable flag to render window.

* examples/gui_button.cpp: Match the global coordinates used in
ButtonLayoutTest.

* examples/gui_button.cpp: Implement workaround fix to keep
button_ex0's font to stick around as the BitmapFont object
we set initially (VIII.png).

* examples/gui_listbox.cpp: Drop render window's resolution down to
640x480 (was 800x600) to match ButtonLayoutTest. This is officially
done simply to preserve ease of comparing screen-shots, but the
resolution drop also has two other important reasons: a) helps
performance considerably on older systems (windev); b) hopefully
helps ease porting to mobile platforms down the road.

* examples/gui_listbox.cpp: Add resizable flag to render window.

* examples/gui_listbox.cpp: Match the global coordinates used in
ListBoxLayoutTest.