src/video_output.c: cleanup vout_GetPictureCount()

Use define VOUT_MAX_PICTURES from vout_internal.h instead of fixed number.

src/video_output/video_output.c

@@ -470,7 +470,7 @@ void vout_HoldPicture(vout_thread_t *vout, picture_t *picture)
 int vout_GetPictureCount(vout_thread_t *vout)
 {
     if (vout == NULL)
-        return 20; /* FIXME: vout_internal.c has VOUT_MAX_PICTURES set to 20 */
+        return VOUT_MAX_PICTURES; /* FIXME: vout_internal.h has VOUT_MAX_PICTURES set to 20 */
 
     vlc_mutex_lock(&vout->p->picture_lock);
     int size = picture_pool_GetSize(vout->p->decoder_pool);