[css-masking] Deprecating 'clip'

  # Note: With this specification the clip property is deprecated.

Deprecation is a normative statement, so shouldn't be in a note.

  # <shape>
  #
  # In CSS 2.1, the only valid <shape> value is: rect(<top>, <right>, <bottom>, <left>)

This isn't the CSS2.1 spec.

Also, <shape> seems overly broad for something that expands only
to rect(). I think we should change this type's name here and in
CSS2.1 to something else (<clip-rect>?) and allow Basic Shapes
to define <shape> for use everywhere else. It's very clumsy for
<shape> to only define rectangles defined by two points and
<basic-shape> to have much broader expressiveness than <shape>.

~fantasai