Utility methods for Val (bevyengine#6134)

# Objective

Adds a better interface for performing mathematical operations with UI unit `Val`. Fixes bevyengine#6080.

## Solution

- Added `try_add` and `try_sub` methods to Val.
- Removed the `Add` and `AddAssign` impls for `Val` that introduced unintuitive and bug-prone behaviour.
- As a consequence of the prior,  ~~changed the `Add` and `Sub` impls for the `Size` struct to take a `(Val, Val)` instead of `Vec2`~~ deleted the `Add` and `Sub` impls for the `Size` struct
- Added a `From<(Val, Val)>` impl for the `Size` struct
- Added `evaluate(size: f32)` method that converts from `Val::Percent` to `Val::Px`.
- Added `try_add_with_size` and `try_sub_with_size` methods to `Val`, which evaluate `Val::Percent` values into `Val::Px` values before adding.

---

## Migration Guide

Instead of using the + and - operators, perform calculations on `Val`s using the new `try_add` and `try_sub` methods. Multiplication and division remained unchanged. Also, when adding or subtracting from `Size`, ~~use a `Val` tuple instead of `Vec2`~~ perform the addition on `width` and `height` separately.


Co-authored-by: Dawid Piotrowski <41804418+Pietrek14@users.noreply.github.com>

crates/bevy_ui/Cargo.toml

@@ -34,3 +34,4 @@ taffy = "0.1.0"
 serde = { version = "1", features = ["derive"] }
 smallvec = { version = "1.6", features = ["union", "const_generics"] }
 bytemuck = { version = "1.5", features = ["derive"] }
+thiserror = "1.0.37"

crates/bevy_ui/src/geometry.rs

@@ -1,7 +1,6 @@
 use crate::Val;
-use bevy_math::Vec2;
 use bevy_reflect::Reflect;
-use std::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Sub, SubAssign};
+use std::ops::{Div, DivAssign, Mul, MulAssign};
 
 /// A type which is commonly used to define positions, margins, paddings and borders.
 ///
@@ -356,42 +355,15 @@ impl Size {
     };
 }
 
-impl Add<Vec2> for Size {
-    type Output = Size;
-
-    fn add(self, rhs: Vec2) -> Self::Output {
-        Self {
-            width: self.width + rhs.x,
-            height: self.height + rhs.y,
-        }
-    }
-}
-
-impl AddAssign<Vec2> for Size {
-    fn add_assign(&mut self, rhs: Vec2) {
-        self.width += rhs.x;
-        self.height += rhs.y;
-    }
-}
-
-impl Sub<Vec2> for Size {
-    type Output = Size;
-
-    fn sub(self, rhs: Vec2) -> Self::Output {
+impl From<(Val, Val)> for Size {
+    fn from(vals: (Val, Val)) -> Self {
         Self {
-            width: self.width - rhs.x,
-            height: self.height - rhs.y,
+            width: vals.0,
+            height: vals.1,
         }
     }
 }
 
-impl SubAssign<Vec2> for Size {
-    fn sub_assign(&mut self, rhs: Vec2) {
-        self.width -= rhs.x;
-        self.height -= rhs.y;
-    }
-}
-
 impl Mul<f32> for Size {
     type Output = Size;
 
@@ -433,27 +405,16 @@ mod tests {
     use super::*;
 
     #[test]
-    fn test_size_add() {
-        assert_eq!(
-            Size::new(Val::Px(10.), Val::Px(10.)) + Vec2::new(10., 10.),
-            Size::new(Val::Px(20.), Val::Px(20.))
-        );
+    fn test_size_from() {
+        let size: Size = (Val::Px(20.), Val::Px(30.)).into();
 
-        let mut size = Size::new(Val::Px(10.), Val::Px(10.));
-        size += Vec2::new(10., 10.);
-        assert_eq!(size, Size::new(Val::Px(20.), Val::Px(20.)));
-    }
-
-    #[test]
-    fn test_size_sub() {
         assert_eq!(
-            Size::new(Val::Px(20.), Val::Px(20.)) - Vec2::new(10., 10.),
-            Size::new(Val::Px(10.), Val::Px(10.))
+            size,
+            Size {
+                width: Val::Px(20.),
+                height: Val::Px(30.),
+            }
         );
-
-        let mut size = Size::new(Val::Px(20.), Val::Px(20.));
-        size -= Vec2::new(10., 10.);
-        assert_eq!(size, Size::new(Val::Px(10.), Val::Px(10.)));
     }
 
     #[test]