sd_lavc: implement --sub-pos for bitmap subtitles

Simple enough to do. May have mixed results. Typically, bitmap subtitles
will have a tight bounding box around the rendered text. But if for
example there is text on the top and bottom, it may be a single big
bitmap with a large transparent area between top and bottom. In
particular, DVD subtitles are really just a single screen-sized
RLE-encoded bitmap, though libavcodec will crop off transparent areas.

Like with sd_ass, you can't move subtitles _down_ if they are already in
their origin position. This could probably be improved, but I don't want
to deal with that right now.

DOCS/man/options.rst

@@ -1990,6 +1990,9 @@ Subtitles
     :strip: Radically strip all ASS tags and styles from the subtitle. This
             is equivalent to the old ``--no-ass`` / ``--no-sub-ass`` options.
 
+    This also controls some bitmap subtitle overrides, as well as HTML tags in
+    formats like SRT, despite the name of the option.
+
 ``--sub-ass-force-margins``
     Enables placing toptitles and subtitles in black borders when they are
     available, if the subtitles are in the ASS format.

sub/sd_lavc.c

@@ -447,6 +447,23 @@ static void get_bitmaps(struct sd *sd, struct mp_osd_res d, int format,
         w = priv->video_params.w;
         h = priv->video_params.h;
     }
+
+    if (opts->sub_pos != 100 && opts->ass_style_override) {
+        int offset = (100 - opts->sub_pos) / 100.0 * h;
+
+        for (int n = 0; n < res->num_parts; n++) {
+            struct sub_bitmap *sub = &res->parts[n];
+
+            // Decide by heuristic whether this is a sub-title or something
+            // else (top-title, covering whole screen).
+            if (sub->y < h / 2)
+                continue;
+
+            // Allow moving up the subtitle, but only until it clips.
+            sub->y = MPMAX(sub->y - offset, 0);
+        }
+    }
+
     osd_rescale_bitmaps(res, w, h, d, video_par);
 }