Primitive::Text.bounds has semantic meaning depending on alignment

[twitchyliquid64]

Primitive::Text is defined as:

    Text {
        /// The contents of the text
        content: String,
        /// The bounds of the text
        bounds: Rectangle,
        /// The color of the text
        color: Color,
        /// The size of the text
        size: f32,
        /// The font of the text
        font: Font,
        /// The horizontal alignment of the text
        horizontal_alignment: HorizontalAlignment,
        /// The vertical alignment of the text
        vertical_alignment: VerticalAlignment,
    },

Based on the doc comments, I would assume that bounds.x & bounds.y refer to the bounding box top-left, and width+x + height+y refer to the bottom right.

This is only true if alignment is set to Left & Top:

        let x = match horizontal_alignment {
            iced_native::HorizontalAlignment::Left => bounds.x,
            iced_native::HorizontalAlignment::Center => bounds.center_x(),
            iced_native::HorizontalAlignment::Right => bounds.x + bounds.width,
        };

        let y = match vertical_alignment {
            iced_native::VerticalAlignment::Top => bounds.y,
            iced_native::VerticalAlignment::Center => bounds.center_y(),
            iced_native::VerticalAlignment::Bottom => bounds.y + bounds.height,
        };

Hence, the bounds X/Y change, mapping to the input X/Y position that the wgpu backend expects for text of that alignment.

This has proven very challenging to work around for the raqote backend, which always needs the top-left X/Y position.

Could we either:

1. Change the semantics so bounds is always the bounding box, or
2. Provide the information from TextBackend::measure() in the primitive, so its easier to recompute the correct position (rather than either caching text runs in the backend or re-adding all the glyph sizes)

[hecrj]

I talked a bit about this in #377 (review).

For a raqote backend, I believe it may be interesting to not use Primitive at all and instead implement the widget Renderer traits directly. We can find and discuss any similarities later.