Set a new playlist
#include <mm/renderer.h>
int mmr_list_change(mmr_context_t *ctxt, const char *url, int delta)
Set a new playlist without interrupting playback. This function can be used only during playback of a playlist (including when it's paused, but not stopped).
The new playlist must contain the currently playing track at position n+delta, where n is its position on the old playlist and delta is the difference in its position in the two lists. This last value is signed, so it can be negative.
Zero on success, -1 on failure (use mmr_error_info())