It's not needed as picture flow has it's own buffer.
This reverts commit 9076b433d18b5db1a1987fe99ca7c70808f22b0e.
Detailed explanation from Thomas Martiz (thanks!):
buflib buffers can be passed to yielding functions just fine. Problems
only arise if the are concurrent allocations, for example if two threads
allocate from the same context simultaneously or if the callee does it's
own allocations. This can't happen in the pictureflow case, it has it's
own context and a single thread allocating from it.
Therefore the problem isn't yield() itself, but possible concurrent
buflib_alloc() calls that result from the thread switch. This is because
compaction only ever happens on allocation (and not in a backgroud
thread or so).