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.
This allows the insertion of multiple empty static blocks for the purpose of efficiently bringing a stream to a byte boundary.