Align the chunk size in the header of the CPR with the actual chunk size (issue #12)

[ColinPitrat]

The CPR doesn't work in some emulators because it is invalid. It advertises a chunk size of 16kB in the header, but the chunk is actually 20 bytes shorter (the size of the CPR + chunk header).

That's because the PAD_TO_16K applies to the whole output: CPR header + content. There are two options:

• modify the size in the header to remove 20 bytes (i.e. 3FEC instead of 4000)
• make the final size 16k + 20 bytes, this is the solution I chose