Rewrite of format audio filter.

 It factorizes the common code and implements all conversions between s8, u8,
s16l/b, u16l/b, s24l/b, s32l/b and fl32 (close #3162).
 In cases a direct conversion is not implemented, s16 is used as the middle
format.
 The following direct conversions have been removed: fl32->s24, fl32->u16, u16->fl32.