Register filters as resources when
instantiated by stream_filter_(ap|pre)pend().
Export php_stream_filter_flush() internal function to wind buffered data
out of a particular filter until consumed by a later filter or sent to
stream->readbuffer or stream->ops->write()
This breaks user-space filters (for the time being), and those
weird convert.* filters in ext/standard/filters.c
The filters stack has been separated into one chain for read and one chain
for write.
The user-space stream_filter_append() type functions currently only operate
on the read chain. They need extending to work with the write chain too.
