[css-images] more midpoint issue

While writing the validation checker for gradient midpoints, I ran into an
issue: since positions can be defined as a mixture of length units and
percentages, it's impossible to say that gradient midpoints are invalid.

Maybe we can say that if a midpoint is in an invalid state, it should be
ignored for rendering instead?
Invalid would be if the midpoint is positioned:
- at the beginning or the end of the grdient line
- next to another midpoint
- on top of a regular colorstop