[rbp] Hardware accelerated resampling

This replaces the format conversion, up/down mixing and resampling code from ActiveAE with a GPU accelerated version.
Should significantly reduce CPU when using paplayer or dvdplayer.

Requires updated firmware
[ffmpeg] Speed up wtv index creation

The index creation is O(N^2) with number of entries (typically thousands).
On a Pi this can take more than 60 seconds to execute for a recording of a few hours.

By replacing with an O(N) loop, this takes virtually zero time