Make position: fixed a full stacking context

[gsnedders]

From https://lists.w3.org/Archives/Public/www-style/2015Jul/0032.html:

• RESOLVED: Make position: fixed a full stacking context
• Rossen will write the text change in place it in the Positioning spec. Bert will then take the same text and add it as an errata to CSS 2.

CSS 2.1 errata: https://www.w3.org/Style/css2-updates/REC-CSS2-20110607-errata.html#s.9.9.1

This appears to match CSS Positioned Layout, at least.

[Loirooriol]

The CSS Position changes were done in #451