[css-color-4] Use the phrase "prepare both colors for conversion" seems wrong in color interpolation algorithm

In step 2 of the [color interpolation algorithm](https://drafts.csswg.org/css-color-4/#interpolation), it states: > 2. prepare both colors for conversion. this changes any [powerless](https://drafts.csswg.org/css-color-4/#powerless-color-component) components to [missing](https://drafts.csswg.org/css-color-4/#missing-color-component) values. While it doesn't link to the definition, ["prepare a color col1 for conversion"](https://drafts.csswg.org/css-color-4/#prepare-a-color-col1-for-conversion), the wording, and lack of other definition seem to imply that is what it is referring to. But, the ["prepare a color col1 for conversion"](https://drafts.csswg.org/css-color-4/#prepare-a-color-col1-for-conversion) seems wrong here. The second step of ["prepare a color col1 for conversion"](https://drafts.csswg.org/css-color-4/#prepare-a-color-col1-for-conversion) states: > 2. If src is in a [cylindrical polar color](https://drafts.csswg.org/css-color-4/#cylindrical-polar-color) representation, convert col1 to the corresponding [rectangular orthogonal color](https://drafts.csswg.org/css-color-4/#rectangular-orthogonal-color) representation and let this be the new col1. and we certainly don't want to do that before interpolation. Maintaining [cylindrical polar color](https://drafts.csswg.org/css-color-4/#cylindrical-polar-color) representation is one of the features of interpolation via the hue interpolation modes. It seems the intent was probably just to apply the "Change any [powerless components](https://drafts.csswg.org/css-color-4/#powerless-color-component) in src to [missing components](https://drafts.csswg.org/css-color-4/#missing-color-component)", but its getting more than it bargained for. If that is the intent, probably the easiest edit is just to state that explicitly, especially since that would remove the need for the clarifying second part. cc @svgeesus