Rename mul_alpha to multiply_alpha?

[waywardmonkeys]

This would make it match the API in Peniko, not just for color but brushes and gradients. Otherwise, Vello might end up with code like:

        skrifa::color::Brush::Solid {
            palette_index,
            alpha,
        } => color_index(cpal, palette_index)
            .map(|it| Brush::Solid(it.mul_alpha(alpha)))
            .unwrap_or(foreground_brush.to_owned().multiply_alpha(alpha)),