fn decode_fixed(bytes: &[u8], power_ten: i32) -> Option<Fixed>
Decode a fixed point value, scaling to a specific power of ten.
See https://gitlab.freedesktop.org/freetype/freetype/-/blob/80a507a6b8e3d2906ad2c8ba69329bd2fb2a85ef/src/psaux/psconv.c#L195