filter_chain: use a vlc_list for chaining filters
- The node storage corresponds to the next/prev on an element.
- The filter_list storage corresponds to the head/tail of the list.
The vlc_list
allows forward and backward iteration, even combining the two. It can remove an element faster than the local code and has API's to get the first/last elements or check if it's empty.
Remove some functions that can be handled with other API's.