Undefined and Auto should not have equivalent behavior

[alice-i-cecile]

taffy version

0.3

Context

          Undefined and Auto definitely aren't the same. 

Ultra minimal example, red window:

use bevy::prelude::*;
fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_startup_system(|mut commands: Commands| {
            commands.spawn(Camera2dBundle::default());
            commands.spawn(NodeBundle {
                style: Style {
                    size: Size::new(Val::Percent(100.), Val::Auto),
                    ..Default::default()
                },
                background_color: BackgroundColor(Color::RED),
                ..Default::default()
            });
        })
        .run();
}

Grey window:

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_startup_system(|mut commands: Commands| {
            commands.spawn(Camera2dBundle::default());
            commands.spawn(NodeBundle {
                style: Style {
                    size: Size::new(Val::Percent(100.), Val::Undefined),
                    ..Default::default()
                },
                background_color: BackgroundColor(Color::RED),
                ..Default::default()
            });
        })
        .run();

The Taffy 2.x compute source is full of predicates like child_style.cross_size(constants.dir) == Dimension::Auto etc.

Originally posted by @ickshonpe in bevyengine/bevy#7475 (comment)

[nicoburns]

I believe that Undefined and Auto should have equivalent behaviour (see https://developer.mozilla.org/en-US/docs/Web/CSS/width#formal_definition which says that the "initial" (i.e. default) value if width is auto). As I understand it, the problem is that in Taffy 0.2, Undefined and Auto do not have equivalent behaviour (although it's quite close). I believe this is already fixed in Taffy 0.3 by #269 which removed Dimension::Undefined entirely. Although I may have caused some confusion on the bevy issues because I thought that PR was already included in Taffy 0.2 and it turns out it isn't.