doc/filters: add a curves example with psfile.
vf_histogram: avoid floats, unbreak fate

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
sws/x86: update RENAME(rgb24toyv12)() to using the user provided rgb2yuv table

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>