Rewrite fill_default_ref_list(), the old code was obfuscated beyond
repair with hacks.
new code is ~60lines old was ~200
Fixes at least:
FRExt/HCHP2_HHI_A.264
one sample also get decoded much better:
FRExt/FRExt1_Panasonic.avc (PSNR 11 -> 80)
(no i do not know why, the old code was too a big mess to figure out
 what it did)

Originally committed as revision 14398 to svn://svn.ffmpeg.org/ffmpeg/trunk
