Use exact sRGB matrix values.

https://photosauce.net/blog/post/what-makes-srgb-a-special-color-space
alerted me to that sRGB does not use the exact same matrix as Rec. 709;
it demands specific, rounded-off values. The difference should be minute,
but now we are at least exact in the forward direction.

Note that this isn't an ABI break per se, even though the definition of the
COLORSPACE_sRGB enum changes; old software will simply still continue
to treat it as Rec. 709, while newly-compiled software will get the
corrected version.