fate: add xBR filter tests
avfilter/vf_lut: gammaval709()

See http://www.itu.int/rec/R-REC-BT.709
Item 1.2, overall opto-electronic transfer characteristics at source

Signed-off-by: Peter Ross <pross@xvid.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>