avidemux: add new index parsing code

avidemux: add new index parsing code

Add a new function and datastructure to parse and hold the index entries on a
per stream base. Also avoid doing too much work trying to figure out the
timestamps and durations as we can trivially do that later.

Less information in the entries makes them 2 times smaller and not doing too
much work makes this code about 12 times faster than the regular case.

Hook in the new function alongside the existing function for comparison until
the rest of the code is updated to handle the new index datastructure.