spudec: restore --sub-forced-only support

This was broken with 84829a4 "Merge branch 'osd_changes' into master".
The new OSD/subtitle code never respected the --sub-forced-only option,
and the old code containing the code for this was removed in fd5c4a1.

sub/spudec.c

@@ -629,6 +629,8 @@ int spudec_visible(void *this){
 	     spu->now_pts < spu->end_pts &&
 	     spu->height > 0);
 //    printf("spu visible: %d  \n",ret);
+    if ((spu->forced_subs_only) && !(spu->is_forced_sub))
+        ret = 0;
     return ret;
 }