Re-export app_units::Au or disable serde for app_units

In order to create a AddFontInstance struct, you have to use an app_units::Au. This is very annoying because:

• app_units depends on serde and num_traits, both very compile-time heavy crates that are completely unused, yet they still need to be compiled in
• This is the only place in the entire public webrender API where Au is used
• app_units isn't re-exported, so other crates have to add the exact version that webrender uses to their Cargo.toml manually.

So my proposal is to create a function that creates an Au from an i32 value, this way you don't have to re-export the Au, i.e.:

pub fn webrender_create_au_from_px(value: i32) -> Option<Au> {
    let value = value * AU_PER_PX;
    if value < MIN_AU || value > MAX_AU {
        None
    } else {
        Some(Au(value))
    }
}

And please make serde and num-traits in the app-units crate optional - for a regular webrender application, these crates go completely unused because serde serialization is only used for the debugger, but these crates still have to be compiled, leading to bad compile times.