Don't derive Copy for FontSize/LetterSpacing/LineHeight
Don't derive Copy for FontSize/LetterSpacing/LineHeight

Clippy said this:

warning: methods with the following characteristics: (`to_*` and `self` type is `Copy`) usually take `self` by value
   --> src/font_props.rs:451:20
    |
451 |     pub fn to_user(&self, params: &NormalizeParams) -> f64 {
    |                    ^^^^^
    |
    = help: consider choosing a less ambiguous name
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#wrong_self_convention

as it prefers "to_foo(self) -> Owned" for self types that are Copy.
However, having to_user() appear to consume the self makes me
uncomfortable (even if the type is Copy).

Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/570>

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg