ffmpeg: make -s work for subtitles too.

Some codecs allow to encode the frame size and some players use it.

ffmpeg_opt.c

@@ -1218,6 +1218,16 @@ static OutputStream *new_subtitle_stream(OptionsContext *o, AVFormatContext *oc,
 
     MATCH_PER_STREAM_OPT(copy_initial_nonkeyframes, i, ost->copy_initial_nonkeyframes, oc, st);
 
+    if (!ost->stream_copy) {
+        char *frame_size = NULL;
+
+        MATCH_PER_STREAM_OPT(frame_sizes, str, frame_size, oc, st);
+        if (frame_size && av_parse_video_size(&subtitle_enc->width, &subtitle_enc->height, frame_size) < 0) {
+            av_log(NULL, AV_LOG_FATAL, "Invalid frame size: %s.\n", frame_size);
+            exit_program(1);
+        }
+    }
+
     return ost;
 }
 
@@ -2219,7 +2229,7 @@ const OptionDef options[] = {
     /* video options */
     { "vframes", HAS_ARG | OPT_VIDEO | OPT_FUNC2, {(void*)opt_video_frames}, "set the number of video frames to record", "number" },
     { "r", HAS_ARG | OPT_VIDEO | OPT_STRING | OPT_SPEC, {.off = OFFSET(frame_rates)}, "set frame rate (Hz value, fraction or abbreviation)", "rate" },
-    { "s", HAS_ARG | OPT_VIDEO | OPT_STRING | OPT_SPEC, {.off = OFFSET(frame_sizes)}, "set frame size (WxH or abbreviation)", "size" },
+    { "s", HAS_ARG | OPT_VIDEO | OPT_SUBTITLE | OPT_STRING | OPT_SPEC, {.off = OFFSET(frame_sizes)}, "set frame size (WxH or abbreviation)", "size" },
     { "aspect", HAS_ARG | OPT_VIDEO | OPT_STRING | OPT_SPEC, {.off = OFFSET(frame_aspect_ratios)}, "set aspect ratio (4:3, 16:9 or 1.3333, 1.7777)", "aspect" },
     { "pix_fmt", HAS_ARG | OPT_EXPERT | OPT_VIDEO | OPT_STRING | OPT_SPEC, {.off = OFFSET(frame_pix_fmts)}, "set pixel format", "format" },
     { "bits_per_raw_sample", OPT_INT | HAS_ARG | OPT_VIDEO, {(void*)&frame_bits_per_raw_sample}, "set the number of bits per raw sample", "number" },