Currently priming the zlib decompressor involves compressing
data directly after having decompressed it and decompressing
it again in order to set the "dictionary" and to initialize
the adler32-checksum. Yet this is wasteful and can be simplified
by synthetizing the compressed data via non-compressed blocks.

This reduced the amount of allocations for the decoding part
of fate-vsynth1-flashsv2, namely from
total heap usage: 9,135 allocs, 9,135 frees, 376,503,427 bytes allocated
to
total heap usage: 2,373 allocs, 2,373 frees, 14,144,083 bytes allocated
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>