Fixed position elements can only have the viewport as the containing block

[mrobinson]

Currently Servo assumes that fixed position elements will only have the viewport as the containing block. The spec describes other situations where this may be the case though:

• A transform or perspective value other than none
• A will-change value of transform or perspective
• A filter value other than none or a will-change value of filter (only works on Firefox).

This probably means that Flows will need to carry a fixed containing block link, like they do for absolute containing blocks.