openh264enc: add support for cabac entropy coding

Submitted by Nicola @drakkan

Link to original bug (#757419)

Description

Created attachment 314569
initial patch

openh264 introduced support for cabac entropy coding.

Initial patch to add support for cabac to openh264enc plugin, please review

Patch 314569, "initial patch":
0001-openh264enc-add-support-for-cabac-entropy-coding.patch