codec/h264/h264dec: implement parsing process for total number of non-zero transform coefficient levels and number of trailing ones

[Saxon Milton]

Process is specified as follows in section 9.2.1 of specifications.

NB: Apologies, but you will need to go to see attached pages that provide this section. I could not copy the section here because formatting as is done in the specifications was not possible.

[Saxon Milton]

Resolved in #243