ffprobe: show bitmap subtitles size.

Fix trac ticket #2930.

ffprobe.c

@@ -1711,6 +1711,17 @@ static void show_stream(WriterContext *w, AVFormatContext *fmt_ctx, int stream_i
             print_int("channels",        dec_ctx->channels);
             print_int("bits_per_sample", av_get_bits_per_sample(dec_ctx->codec_id));
             break;
+
+        case AVMEDIA_TYPE_SUBTITLE:
+            if (dec_ctx->width)
+                print_int("width",       dec_ctx->width);
+            else
+                print_str_opt("width",   "N/A");
+            if (dec_ctx->height)
+                print_int("height",      dec_ctx->height);
+            else
+                print_str_opt("height",  "N/A");
+            break;
         }
     } else {
         print_str_opt("codec_type", "unknown");