Fix crash when attempting to use a document that has a 16-bit float XYZ color space

XyzF16ColorSpace::colorDepthId() would return Float32BitsColorDepthID. This caused Krita to segfault when you created a document using the XYZ color model with 16-bit float depth.
It was possible to convert a layer to use the same properties, but an assertion would fail when trying to reopen it.

Differential Revision: https://phabricator.kde.org/D3752