color: structurize parametric curve data

Rather than using a two-dimensional array, provide names to the fields.
The fields are primarily used by their names, but we also need the data
as a flat array of floats, so use unions to achieve that: two different
views into the same data.

This makes the code more self-explanatory. In color-operations.c it
removes a handful of temporaries. Comparison in color-properties.c is
simplified. ARRAY_COPY() turns into an assignment.

MAX_PARAMS_PARAM_CURVE is eliminated. Instead of having to maintain a
copy in fragment.glsl, the definition there is automated.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>