lavc/flac_parser: use a custom FIFO implementation

FLAC parser currently uses AVFifoBuffer in a highly non-trivial manner,
modifying its "internals" (the whole struct is currently public, but no
other code touches its contents directly). E.g. it does not use any
av_fifo functions for reading the FIFO contents, but implements its own.

Reimplement the needed parts of the AVFifoBuffer API in the FLAC parser,
making it completely self-contained. This will allow us to make
AVFifoBuffer private.