[css-ui-4] cursor should use <image> not <url>

[svgeesus]

Issue originally raised on www-style in 2012 by @tabatkins and copied over to tracker by @fantasai

(CSS Images 3 says](https://drafts.csswg.org/css-images-3/#placement)

This module defines and extends the <image> value type defined in [CSS-VALUES-3]. It also replaces the <url> type with <image> in the background-image, cursor, and list-style-image definitions in CSS1 and CSS2

However, CSS UI 4 uses <url>

[ [<url> [<x> <y>]?,]*
[ auto | default | none |
context-menu | help | pointer | progress | wait |
cell | crosshair | text | vertical-text |
alias | copy | move | no-drop | not-allowed | grab | grabbing |
e-resize | n-resize | ne-resize | nw-resize | s-resize | se-resize | sw-resize | w-resize | ew-resize | ns-resize | nesw-resize | nwse-resize | col-resize | row-resize | all-scroll |
zoom-in | zoom-out
] ]

I didn't see this in closed issues, feel free to close if this is a duplicate.

[frivoal]

In the definition of the values for this property, threre's this sentence:

Conforming User Agents may, instead of <url>, support <image> which is a superset.

Do you think we should move away from that, and just use <image>? Conceptually, I'd have no problem with it, but I don't believe that the UAs actually support <image> values other than URLs, so going with more than a may seems maybe a bit optimistic.

[frivoal]

Based on the explanation above. I'm planning to close as wontfix/invalid. Now's the time to disagree if you'd like to stop me.

[svgeesus]

That seems fair enough to me.