rewrite Move_cursor_with_constraints() to clip negative coordinates

also introduce Handle_mouse_btn_change() so we don't bother to
check everything when only the mouse button state changed

src/input.h

@@ -49,7 +49,8 @@ int Has_shortcut(word function);
 /// Adjust mouse sensitivity (and actual mouse input mode)
 void Adjust_mouse_sensitivity(word fullscreen);
 
-int Move_cursor_with_constraints(int);
+int Move_cursor_with_constraints(int x, int y);
+int Handle_mouse_btn_change(void);
 
 ///
 /// This holds the ID of the GUI control that the mouse