fix non-exact text h-alignment

[robtfm]

Objective

when a parent container is auto-sized, text alignments Center and Right don't align to the center and right properly. fix it

Solution

ab_glyph positions return +/- values from an anchor point. we currently transform them to positive values from the min-x of the glyphs, and then offset from the left of the bounds. instead, we can keep the negative values as ab_glyph intended and offset from the left/middle/right of the bounds as appropriate.

Testing

texts with align left, center, right, all contained in the purple boxes:
before (0.14.0-rc.2):

after:

code:

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .run();
}

fn setup(mut commands: Commands) {
    commands.spawn(Camera2dBundle::default());

    for (left, justify) in [
        (100.0, JustifyText::Left),
        (500.0, JustifyText::Center),
        (900.0, JustifyText::Right),
    ] {
        commands
        // container
        .spawn(NodeBundle {
            style: Style {
                flex_direction: FlexDirection::Column,
                position_type: PositionType::Absolute,
                left: Val::Px(left),
                top: Val::Px(100.0),
                width: Val::Px(300.0),
                ..Default::default()
            },
            ..Default::default()
        })
        .with_children(|commands| {
            commands.spawn(NodeBundle{
                style: Style {
                    flex_direction: FlexDirection::Row,
                    height: Val::Px(75.0),
                    ..Default::default()
                },
                background_color: Color::srgb(1.0, 0.0, 1.0).into(),
                ..Default::default()
            }).with_children(|commands| {
                // a div that reduces the available size
                commands.spawn(NodeBundle {
                    style: Style {
                        width: Val::Px(75.0),
                        ..Default::default()
                    },
                    background_color: Color::srgb(0.0, 1.0, 0.0).into(),
                    ..Default::default()
                });

                // text with width=auto, but actual size will not be what it expcets due to the sibling div above
                commands.spawn(TextBundle {
                    text: Text::from_section("Some text that wraps onto a second line", Default::default()).with_justify(justify),
                    style: Style {
                        align_self: AlignSelf::Center,
                        ..Default::default()
                    },
                    ..Default::default()
                });
            });
        });
    }
}

[robtfm]

i haven't tested subpixel offsets explicitly (not really sure what i'm looking for), but i did floor the anchor so that the new position will be a whole number of pixels from where it was originally. hopefully that works.