Sprite: allow using a sub-region (Rect) of the image (bevyengine#6014)

Very small change that improves the usability of `Sprite`.

Before this PR, the only way to render a portion of an `Image` was to create a `TextureAtlas` and use `TextureAtlasSprite`/`SpriteSheetBundle`. This can be very annoying for one-off use cases, like if you just want to remove a border from an image, or something. Using `Sprite`/`SpriteBundle` always meant that the entire full image would be rendered.

This PR adds an optional `rect` field to `Sprite`, allowing a sub-rectangle of the image to be rendered. This is similar to how texture atlases work, but does not require creating a texture atlas asset, making it much more convenient and efficient for quick one-off use cases.

Given how trivial this change is, it really felt like missing functionality in Bevy's sprites API. ;)

## Changelog

Added:
 - `rect` field on `Sprite`: allows rendering a portion of the sprite's image; more convenient for one-off use cases, than creating a texture atlas.

crates/bevy_sprite/src/render/mod.rs

@@ -257,8 +257,7 @@ pub fn extract_sprites(
             entity,
             color: sprite.color,
             transform: *transform,
-            // Use the full texture
-            rect: None,
+            rect: sprite.rect,
             // Pass the custom size
             custom_size: sprite.custom_size,
             flip_x: sprite.flip_x,

crates/bevy_sprite/src/sprite.rs

@@ -1,5 +1,5 @@
 use bevy_ecs::component::Component;
-use bevy_math::Vec2;
+use bevy_math::{Rect, Vec2};
 use bevy_reflect::Reflect;
 use bevy_render::color::Color;
 
@@ -15,6 +15,9 @@ pub struct Sprite {
     /// An optional custom size for the sprite that will be used when rendering, instead of the size
     /// of the sprite's image
     pub custom_size: Option<Vec2>,
+    /// An optional rectangle representing the region of the sprite's image to render, instead of
+    /// rendering the full image. This is an easy one-off alternative to using a texture atlas.
+    pub rect: Option<Rect>,
     /// [`Anchor`] point of the sprite in the world
     pub anchor: Anchor,
 }