XZ writes filters in the order that they are used during compression, so
we need to process them in the reverse order while decompression.
This wasn't noticed earlier because we only supported the LZMA2 filter.
The constructor is now only concerned with creating the required
streams, which means that it no longer fails for XZ streams with
invalid headers. Instead, everything is parsed and validated during the
first read, preparing us for files with multiple streams.
