command: extend osd-overlay command with bounds reporting

This is more or less a minimal hack to make _some_ text measurement
functionality available to scripts. Since libass does not support such a
thing, this simply uses the bounding box of the rendered text.

This is far from ideal. Problems include:
- using a bitmap bounding box
- additional memory waste and/or flushing caches
- dependency on window size
- odd small deviations with different window sizes (run osd-test.lua and
  resize the window after each timer update; the bounding boxes aren't
  adjusted in an overly useful way)
- inability to query the size _after_ actual rendering

But I guess it's a start. Since I'm aware that it's crap, add a threat
to the manpage that this may be changed/removed again. For now, I'm
interested whether anyone will have use for it in its current form, as
it's an often requested feature.

sub/osd.h

@@ -209,6 +209,9 @@ struct osd_external_ass {
     char *data;
     int res_x, res_y;
     int z;
+    bool hidden;
+
+    double *out_rc; // hack to pass boundary rect, [x0, y0, x1, y1]
 };
 
 // defined in osd_libass.c and osd_dummy.c