Gapless test set

Each folder holds one continuous tone: a sine sweep from 220 Hz up to 880 Hz and
back down, about 24 seconds long, at -20 dBFS. It is cut into 2 to 5 files at
arbitrary sample positions and each file is encoded on its own.

Played in order without a gap, it sounds like one smooth sweep. Any gap, dropped
or repeated sample at a file boundary is an audible click.

  FLAC-44-16              FLAC, 44.1 kHz / 16 bit, 5 files
  FLAC-96-24              FLAC, 96 kHz / 24 bit, 3 files
  MP3-44-V2               MP3, 44.1 kHz, VBR (LAME -V 2), 4 files
  WAV-44-16               WAV, 44.1 kHz / 16 bit, 3 files
  MP3-44-V2-META          MP3-44-V2, stuffed with metadata, 4 files

MP3 is gapless only by the encoder delay and padding in each file's LAME
header; a player that ignores the header leaves a short silence at every
boundary.

The -META folder is the same cut with what taggers leave in real files:
cover art from 2 KB to 2 MB, lyrics, one or two ID3v2 tags in front of the
audio, an APEv2 tag with its own cover and ID3v1 behind it. A player has to
skip all of it at both ends of a file and still join the pieces without a
gap.

Copy the GAPLESS folder to the player, open a folder and play 01, or play the
whole folder. Start with the volume low: the tone is a pure sine.

The files are deterministic: the generator (gen_gapless.py) checks that the
pieces decode back to the uncut sweep, sample for sample (for MP3: to its
exact length, within the coding noise).
