Mousing over button text shouldn't show cursor=text

[frewsxcv]

Comparing Servo and Chrome on this page:

[jdm]

This comes from mouse_over in components/layout/query.rs, but originates from the code in display_list_builder.rs which stores cursors for text fragments in build_display_list_for_text_fragment and build_display_items_for_selection_if_necessary. I guess we either need a way of overriding that or explicit checks to see if the text fragment is in a button. I guess we could try adding a cursor property to the button rules in the UA stylesheet, though?

[frewsxcv]

This might be the associated line in Gecko?

https://github.com/mozilla/gecko-dev/blob/e192050ec9987883ea76c85950301dd5c4314a0c/layout/style/forms.css#L628