[client] ds: detect when the cursor exits into an overlapping window

This adds a new method to the display server interface to allow the
application to notify the ds when there is a guest cursor position
update along with the translated local guest cursor position. This makes
it possible for the display server to keep the local cursor position in
sync with the guest cursor so that window leave events can be detected
when the cursor would move into an overlapping window.

Wayland currently just has a stub for this, and the X11 implementation
still needs some minor tweaking.

client/include/app.h

@@ -37,6 +37,7 @@ LG_MsgAlert;
 
 bool app_isRunning(void);
 bool app_inputEnabled(void);
+bool app_isCaptureMode(void);
 void app_updateCursorPos(double x, double y);
 void app_updateWindowPos(int x, int y);
 void app_handleResizeEvent(int w, int h, double scale, const struct Border border);