This allows deflate to generate the same output when continuing after
a Z_SYNC_FLUSH vs. using deflateSetDictionary() after a Z_FULL_FLUSH
or a deflateReset(). It also slightly improves compression when
flushing by providing two more strings to possibly match at the start
of the new block.
Previously, the bit buffer would hold 1 to 16 bits after "all" of the
output is provided after a Z_BLOCK deflate() call. Now at most seven
bits remain in the output buffer after Z_BLOCK. flush_pending() now
flushes the bit buffer before copying out the byte buffer, in order
for it to really flush as much as possible.
Z_PARTIAL_FLUSH would sometimes emit two empty static blocks instead
of one in order to provide enough lookahead for inflate to be able
to decode what was last compressed. inflate no longer needs that
much lookahead, so this removes the possibility of emitting the
second empty static block. Z_PARTIAL_FLUSH will now emit only one
empty static block.
