These share the same transfer function which is therefore a separate
struct instead of each sharing an implementation only. Strictly
speaking, the RgbSpace defined by Rec.709 could be shared with the sRGB
color space as it uses the exact same primaries and whitepoint but the
advantage due to the difference in transfer function seems negligible.

Uses the precise floating point coefficients and ensure output values
are clamped into the allowed signal range as specified. It already adds
the quantization conversion for 10-bit output without adding the actual
quantization itself because the type representation is not yet clear.

This is for scene linear light, which does not depend on viewing
parameters in the transfer function. For PQ there are is no universal
reversible transfer function given, the recommendation only describes a
set of OETF and EOTF with which a reference viewing environment can
produce the proper luminance but this is not easily reversible.

No more duplicate `from_f64` method, remove Into/From impls and fix the
Luma conversion to use Rgb for now.

This is a separate commit so it can be reintroduced later.