Convert "rate" variable to float everywhere

Convert "rate" variable to float everywhere

This follows the historical LibVLC semantic (and new --rate one), so
bigger is faster. In the process, simplify a few code paths, especially
those in LibVLC, but make many other code paths more complicated due to
integer<->float conversion. Most of those paths could probably be
simplified.

 * Fix RC "fastforward" command behaviour (it was slowing down instead)
 * Fix str_format_mera 'R' format string with negative rates and
   rates such that (rate % 1000 < 100).