Add initial support for 12-bit color mode.

Patch by Janusz Krzysztofik jkrzyszt tis icnet pl
Original thread:
Subject: [FFmpeg-devel] [PATCH v2] Add initial support for 12-bit color mode.
Date: Mon, 1 Mar 2010 02:05:07 +0100

Originally committed as revision 22220 to svn://svn.ffmpeg.org/ffmpeg/trunk