Fix relative font sizes.

The `ComputeFontSizeCb` callback is supposed to return absolute font size, but
returned relative font sizes in many cases.
Derive Eq on all values
Add clear
Factor out CssStylesheetParams and add parse_style_attribute.
Include style attributes in selector matching.
Update tests for new select_style signature.
Update to latest Rust
Add Cm postfix(Length)
Add testcase Cm/Em postfix