kdtest: Fix colorspace conversion for block coeffs.

Makes it consistent with how the conversion is done in prop.c.
kdtest: Fix print_tuple function.

Although the double-pointer indirection is strictly
unnecessary, it matches up with the one in kdtree.c
and allows for printing node values if needed.
kdtest: Allow configurable YCrCb coefficients.
prop: Even better defaults for coeffs.

Use BGR-2-9-5. 4.4% more accurate, 3.3% faster than old default.
Wins all around.