playloop: use a 16:9 ratio with --force-window

ca2b05c changed the window size with --force-window and no video
tracks to be closer to 16:9, but I don't see why we shouldn't have an
actual 16:9 ratio. The advantage is that subtitles with fullscreen and
no video tracks will have the same size and position (depending on the
values of --sub-scale-with-window and --sub-use-margins) as with 16:9
videos, because there will be no (invisible) black bars.

player/playloop.c

@@ -1030,8 +1030,12 @@ int handle_force_window(struct MPContext *mpctx, bool force)
                 break;
             }
         }
+
+        // Use a 16:9 aspect ratio so that fullscreen on a 16:9 screen will not
+        // have vertical margins, which can lead to a different size or position
+        // of subtitles than with 16:9 videos.
         int w = 960;
-        int h = 480;
+        int h = 540;
         struct mp_image_params p = {
             .imgfmt = config_format,
             .w = w,   .h = h,