[css-sizing-3][css-images-3] Distinguish intrinsic size's two definitions with two terms #4961

We have used the term “intrinsic size” to refer to two slightly different concepts (that overlap) in our spec:

• the inherent size of a replaced element (which sometimes exists)
• min-content and max-content sizes (which always exist, and are derived from the content)

We need to have separate terms for these so there's no confusion.