Support 16K sequence coding

Fixed a couple of variable/function definitions, as well as header
handling to support 16K sequence coding at high bit-rates.

The width and height are each specified by two bytes in the header.
Use an extra byte to explicitly indicate the scaling factors in
both directions, each ranging from 0 to 15.

Tested coding up to 16400x16400 dimension.

Change-Id: Ibc2225c6036620270f2c0cf5172d1760aaec10ec