ui_context.rs

    /// Calls closure with a texture constructor enabled.
    /// This is required to render characters.
    pub fn with_texture_constructor<'a>(&mut self, texture_constructor: TextureConstructor<'a, T>, f: |&mut UiContext<T>|) {
        use std::mem::transmute;

        self.glyph_cache.texture_constructor =
            Some(unsafe { transmute(texture_constructor) });
        f(self);
        self.glyph_cache.texture_constructor = None;
    }