Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>

Reduce duplicate code.

Addresses Mantis ticket 0004015.

[mk: use decoder_read_full() only when needed, and a few formal
changes]

Works around WAV stream playback bug, because libaudiofile does not
like partial reads (Mantis 0004028).

Move code from the "mad" plugin.

Move code from the "mad" plugin.

.. instead of InputStream::LockRead(). The former is cancellable.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>

(see https://www.xiph.org/vorbis/doc/v-comment.html for Details)

Remove the "whence" parameter that is not actually necessary, and only
complicates the InputStream implementations.

Allows more compiler optimizations.

Move and refactor code from the Wavpack decoder plugin.

Replaces the method Close().

Fixes missing song length on high-latency Opus files.

According to tests with 320 kbit/s opus files with 60ms packets, we
need to skip up to 29 kB.

This corrects a major mistake from commit 724a59aa - there was one
small thing that commit was supposed to do, and it failed.
AV_TIME_BASE is not a seek flag.