command: export more details about file seekability

If a file is unseekable (consider e.g. a http server without resume
functionality), but the stream cache is active, the player will enable
seeking anyway. Until know, client API user couldn't know that this
happens, and it has implications on how well seeking will work. So add a
property which exports whether this situation applies.

Fixes #1522.

demux/demux.c

@@ -758,6 +758,7 @@ static void demux_copy(struct demuxer *dst, struct demuxer *src)
         dst->file_contents = src->file_contents;
         dst->playlist = src->playlist;
         dst->seekable = src->seekable;
+        dst->partially_seekable = src->partially_seekable;
         dst->filetype = src->filetype;
         dst->ts_resets_possible = src->ts_resets_possible;
         dst->rel_seeks = src->rel_seeks;
@@ -905,6 +906,7 @@ static struct demuxer *open_given_type(struct mpv_global *global,
         if (!in->d_thread->seekable && stream->uncached_stream) {
             mp_verbose(log, "Enabling seeking because stream cache is active.\n");
             in->d_thread->seekable = true;
+            in->d_thread->partially_seekable = true;
         }
         demux_init_cache(demuxer);
         demux_changed(in->d_thread, DEMUX_EVENT_ALL);

demux/demux.h

@@ -185,7 +185,8 @@ typedef struct demuxer {
     int64_t filepos;  // input stream current pos.
     char *filename;  // same as stream->url
     enum demuxer_type type;
-    int seekable; // flag
+    bool seekable;
+    bool partially_seekable; // implies seekable=true
     double start_time;
     // File format allows PTS resets (even if the current file is without)
     bool ts_resets_possible;