[text2png] Respect font's intended line spacing

[wmedrano]

Users can now specify LineHeight::FontSizeRelative (equivalent to previous), LineHeight::Normal, and LineHeight::Absolute.

• LineHeight::Normal was taken from Mozilla CSS line-height doc
• LineHeight::Normal is set as the default.

/// Specifies how line height is determined for text rendering and measurement.
#[derive(Debug, Clone, Default, Copy, PartialEq)]
pub enum LineHeight {
    /// Automatically determine line height from the font's
    /// typographic metrics (`ascent - descent + leading`).
    #[default]
    Normal,
    /// Line height proportional to the font size (e.g., 1.0 means line height equals `font_size`).
    FontSizeRelative(f32),
    /// Explicit line height in pixels.
    Absolute(f32),
}