This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Look up attributes in the "atts" array.  Reduce bloat.

The last piece to allow -ffast-math.

Fixes build problems with mingw32.

Don't allocate each replay_gain_info object on the heap.  Those
objects who held a pointer now store a full replay_gain_info object.
This reduces the number of allocations and heap fragmentation.

The previous patch not only moved code, it also changed the check.
Negative gain values seem to be valid after all, there just was the
"magic" value 0.0 which means "not available".  This patch changes the
"magic" value to "INFINITY", and uses the C99 function isinf() to
check.  It might have been a better idea to use "NAN", but the "NAN"
macro is a GNU extension.

Moved (and renamed) the function calc_replay_gain_scale() to
replay_gain_info.c.

This function determines whether replay gain data is available.

The replay_gain_state struct holds the precalculated scale factor,
which is removed from struct replay_gain_info.

The new command "replay_gain_mode" allows the user to switch the
replay gain mode on-the-fly.  No more mpd.conf editing.