Fix passing of unaligned addresses to dsputil diff_bytes() by

Fix passing of unaligned addresses to dsputil diff_bytes() by
changeing calls to sub_left_prediction such that
the source frame data pointers are no longer offset to skip the initial
elements. This keeps them properly aligned for the subsequent call to diff_bytes.
Changes encoding offset to compensate.

Mailing list thread:
https://lists.mplayerhq.hu/pipermail/ffmpeg-devel/2009-May/069952.html

Originally committed as revision 18919 to svn://svn.ffmpeg.org/ffmpeg/trunk