builtin_shaders.h: update to BT.709-2 coefficients

See https://github.com/FFmpeg/FFmpeg/commit/98038e2ee0c40530c42db70ed3d2f5e0c17b175e and https://en.wikipedia.org/wiki/Rec._709#Luma_coefficients and https://github.com/AviSynth/AviSynthPlus/commit/4df3d53fecac914c596bccb1de05fab1586ddf2d

Not going to affect 8 bit. Those coeff. were used in BT.709-1, that is first version. And I suppose they were used by B. Powell from Canada when he derived the primaries (as matrix coeff. are derived from primaries anyway).