make mouse movement and capture depend on window focus

src/twn_loop.c

@@ -42,6 +42,16 @@ static int event_callback(void *userdata, SDL_Event *event) {
             ctx.resync_flag = true;
             break;
 
+        case SDL_WINDOWEVENT_FOCUS_LOST: {
+            ctx.window_mouse_resident = false;
+            break;
+        }
+
+        case SDL_WINDOWEVENT_FOCUS_GAINED: {
+            ctx.window_mouse_resident = true;
+            break;
+        }
+
         default:
             break;
         }
@@ -706,6 +716,7 @@ static bool initialize(void) {
     */
 
     ctx.render_double_buffered = true;
+    ctx.window_mouse_resident = true;
 
     return true;