[css-view-transitions-1] Use position: absolute instead of position: fixed on ::view-transition

[nt1m]

Fixes #12116

[BorisChiou]

I'm updating the ua stylesheet in Gecko. @noamr @vmpstr would you mind taking a look at this spec change?

[nt1m]

@emilio @BorisChiou If you could create a WPT for this, it would be very appreciated, happy to follow up with a WebKit change too.

[BorisChiou]

I will update pseudo-get-computed-style.html and pseudo-get-computed-style-clean-style.html to make sure the computed value of position of ::view-transition (i.e. getComputedStyle(document.documentElement, "::view-transition").position) is absolute.

UPDATED: in gecko bug 1983941 (https://bugzilla.mozilla.org/show_bug.cgi?id=1983941)