APE reader: style fixes.
APE reader: optimize seeking

* The APE SDK can only decode rather big chunks at a time (several
seconds), so our reader keeps an internal buffer of the last decoded
chunk.
* However, this was always dropped when seeking, meaning the same few
seconds of sound were decoded again and again.
* I'm not sure why MediaPlayer is trying to seek to the position it
already is at, but we can make it work, so why not.

Now the existing buffer is reused if possible when seeking. This makes
it possible to play APE files in MediaPlayer.

Also, several more style fixes.
Add the ape reader to the image again.

Fixes #7048.
AVCodecDecoder: remove annoying assert.

According to Colin this is not supposed to happen, but it does. Since
the affected video otherwise decodes just fine, I think it is safe to
disable the assert and not annoy our users.

Fixes #11409.