Fix visual-bell on X

* src/pgtkterm.c (pgtk_flash): Change size source.

On X, surface_orig is not an image surface but xlib surface, so I
cannot get its size with cairo_image_surface_get_{width,height}.
1 job for feature/pgtk in 65 minutes and 12 seconds (queued for 2 seconds)
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01:05:12