sub: make it easier to set DVD sub decoding with sd_lavc

With this commit, the player will still use spudec.c (the "old" DVD sub
decoder), rather than ffmpeg. But it brings the changes needed to enable
this down to a single line change:

--- a/mplayer.c
+++ b/mplayer.c
@@ -1988,7 +1988,7 @@ static void reinit_subs(struct MPContext *mpctx)
 #endif
         vo_osd_changed(OSDTYPE_SUBTITLE);
     } else if (track->stream) {
-        if (mpctx->sh_sub->type == 'v')
+        if (mpctx->sh_sub->type == 'v' && false)
             init_vo_spudec(mpctx);
         else
             sub_init(mpctx->sh_sub, mpctx->osd);

Also, copy the DVD resolution heuristics from spudec.c (from the
spudec_new_scaled() function). I'm not sure if this is correct or even
needed, but the sd_lavc codd explicitly reverted back to spudec with
code carrying this comment:

    // Assume resolution heuristics only work for PGS and DVB

so it seems likely that the required heuristics were missing, and that
the spudec heuristics may make the DVD compatibility situation at least
as good as with spudec.

Note that it's unlikely that we enable sd_lavc for DVD subs by default,
as there are other problems in combination with direct DVD playback.

mplayer.c

@@ -1795,7 +1795,9 @@ static void update_subtitles(struct MPContext *mpctx, double refpts_tl)
     }
 
     // DVD sub:
-    if (track->vobsub_id_plus_one || type == 'v') {
+    if ((track->vobsub_id_plus_one || type == 'v')
+        && !(sh_sub && sh_sub->active))
+    {
         int timestamp;
         // Get a sub packet from the demuxer (or the vobsub.c thing, which
         // should be a demuxer, but isn't).

sub/dec_sub.c

@@ -39,7 +39,7 @@ void sub_init(struct sh_sub *sh, struct osd_state *osd)
     if (opts->ass_enabled && is_text_sub(sh->type))
         sh->sd_driver = &sd_ass;
 #endif
-    if (strchr("bpx", sh->type))
+    if (strchr("bpxv", sh->type))
         sh->sd_driver = &sd_lavc;
     if (sh->sd_driver) {
         if (sh->sd_driver->init(sh, osd) < 0)

sub/sd_lavc.c

@@ -77,6 +77,26 @@ static void old_avsub_to_spudec(AVSubtitleRect **rects, int num_rects,
     spudec_packet_send(vo_spudec, packet, pts, endpts);
 }
 
+static void guess_resolution(char type, int *w, int *h)
+{
+    if (type == 'v') {
+        /* XXX Although the video frame is some size, the SPU frame is
+           always maximum size i.e. 720 wide and 576 or 480 high */
+        // For HD files in MKV the VobSub resolution can be higher though,
+        // see largeres_vobsub.mkv
+        if (*w <= 720 && *h <= 576) {
+            *w = 720;
+            *h = (*h == 480 || *h == 240) ? 480 : 576;
+        }
+    } else {
+        // Hope that PGS subs set these and 720/576 works for dvb subs
+        if (!*w)
+            *w = 720;
+        if (!*h)
+            *h = 576;
+    }
+}
+
 static int init(struct sh_sub *sh, struct osd_state *osd)
 {
     if (sh->initialized)
@@ -158,8 +178,7 @@ static void decode(struct sh_sub *sh, struct osd_state *osd, void *data,
     if (sub.num_rects > 0) {
         switch (sub.rects[0]->type) {
         case SUBTITLE_BITMAP:
-            // Assume resolution heuristics only work for PGS and DVB
-            if (!osd->support_rgba || sh->type != 'p' && sh->type != 'b') {
+            if (!osd->support_rgba) {
                 if (!vo_spudec)
                     vo_spudec = spudec_new_scaled(NULL, ctx->width, ctx->height,
                                                   NULL, 0);
@@ -214,13 +233,9 @@ static void get_bitmaps(struct sh_sub *sh, struct osd_state *osd,
         priv->outbitmaps = talloc_memdup(priv, priv->inbitmaps,
                                          talloc_get_size(priv->inbitmaps));
     bool pos_changed = false;
-    // Hope that PGS subs set these and 720/576 works for dvb subs
     int inw = priv->avctx->width;
-    if (!inw)
-        inw = 720;
     int inh = priv->avctx->height;
-    if (!inh)
-        inh = 576;
+    guess_resolution(sh->type, &inw, &inh);
     struct mp_eosd_res *d = &osd->dim;
     double xscale = (double) (d->w - d->ml - d->mr) / inw;
     double yscale = (double) (d->h - d->mt - d->mb) / inh;