Update public FFmpeg header files from r16647 to r18286.

Update public FFmpeg header files from r16647 to r18286.

This also includes bumping the version of avutil from 49 to 50, which means the generated libs and build files are also updated.

[scherkus (not reviewing), 2009-03-31 21:44:46]

You can ignore most of /third_party/ffmpeg/include, but take a look at the rest
of the files.

You'll need this patch in order to resolve linker errors with avcodec_thread_*
functions.

[scherkus (not reviewing), 2009-04-01 23:23:05]

You can ignore most of /third_party/ffmpeg/include (diffs between the versions),
but take a look at the rest of the files.

[darin (slow to review), 2009-04-01 23:25:02]

LGTM for chrome changes.