make mouse movement and capture depend on window focus

src/twn_engine_context_c.h

@@ -82,6 +82,8 @@ typedef struct EngineContext {
     bool resync_flag;
     bool was_successful;
     bool render_double_buffered;
+    /* signals mouse focus, used to disable mouse capture */
+    bool window_mouse_resident;
 } EngineContext;
 
 /* TODO: does it need to be marked with TWN_API? */