Revised the Delta filter implementation. The initialization

Revised the Delta filter implementation. The initialization
function is still shared between encoder and decoder, but the
actual coding is in separate files for encoder and decoder.

There are now separate functions for the actual delta
calculation depending on if Delta is the last filter in the
chain or not. If it is the last, the new code copies the
data from input to output buffer and does the delta
calculation at the same time. The old code first copied the
data, then did the delta in the target buffer, which required
reading through the data twice.

Support for LZMA_SYNC_FLUSH was added to the Delta encoder.
This doesn't change anything in the file format.