Support reading packed YUV422 10bit samples from Sveriges Television AB (SVT)

which have AFAIK been created for the jvt:
ftp://vqeg.its.bldrdoc.gov/HDTV/SVT_exports/SVT_YUV10_Exports_/NewMobCal_YUV10_720p5994_/
I have called the format v210x due to its similarity to v210, note though I have
not confirmed that v210x is different from actual v210 samples it just is
different from the description of v210 I am aware of.

Originally committed as revision 18654 to svn://svn.ffmpeg.org/ffmpeg/trunk