x11: implement hidpi scale reporting

(X11 does not support different per-screen DPI (or only via hacks), so
this is pretty simple. If other backends are going to implement this,
then they should send VO_EVENT_WIN_STATE if the DPI for the mpv window
changes by moving it to another screen or such.)

video/out/x11_common.c

@@ -1963,6 +1963,9 @@ int vo_x11_control(struct vo *vo, int *events, int request, void *arg)
         *(double *)arg = fps;
         return VO_TRUE;
     }
+    case VOCTRL_GET_HIDPI_SCALE:
+        *(double *)arg = x11->dpi_scale;
+        return VO_TRUE;
     }
     return VO_NOTIMPL;
 }