Jan Schmidt authored Apr 01, 2020 and Tim-Philipp Müller committed Jun 07, 2020

When running in pull mode (for e.g. mp3 reading),
baseparse currently reads 64KB from upstream, then mp3parse
consumes typically around 417/418 bytes of it. Then
on the next loop, it will read a full fresh 64KB again,
which is a big waste.

Fix the read loop to use the available cache buffer first
before going for more data, until the cache drops to < 1KB.

Fixes #518