Add a size method on Image. (bevyengine#3696)

# Objective

Add a simple way for user to get the size of a loaded texture in an Image object.
Aims to solve bevyengine#3689

## Solution

Add a `size() -> Vec2` method
Add two simple tests for this method.

Updates:
. method named changed from `size_2d` to `size`

crates/bevy_render/src/texture/image.rs

@@ -7,7 +7,7 @@ use crate::{
 };
 use bevy_asset::HandleUntyped;
 use bevy_ecs::system::{lifetimeless::SRes, SystemParamItem};
-use bevy_math::Size;
+use bevy_math::{Size, Vec2};
 use bevy_reflect::TypeUuid;
 use thiserror::Error;
 use wgpu::{
@@ -118,6 +118,14 @@ impl Image {
         self.texture_descriptor.size.height as f32 / self.texture_descriptor.size.width as f32
     }
 
+    /// Returns the size of a 2D image.
+    pub fn size(&self) -> Vec2 {
+        Vec2::new(
+            self.texture_descriptor.size.width as f32,
+            self.texture_descriptor.size.height as f32,
+        )
+    }
+
     /// Resizes the image to the new size, by removing information or appending 0 to the `data`.
     /// Does not properly resize the contents of the image, but only its internal `data` buffer.
     pub fn resize(&mut self, size: Extent3d) {
@@ -440,3 +448,33 @@ impl RenderAsset for Image {
         })
     }
 }
+
+#[cfg(test)]
+mod test {
+
+    use super::*;
+
+    #[test]
+    fn image_size() {
+        let size = Extent3d {
+            width: 200,
+            height: 100,
+            depth_or_array_layers: 1,
+        };
+        let image = Image::new_fill(
+            size,
+            TextureDimension::D2,
+            &[0, 0, 0, 255],
+            TextureFormat::Rgba8Unorm,
+        );
+        assert_eq!(
+            Vec2::new(size.width as f32, size.height as f32),
+            image.size()
+        );
+    }
+    #[test]
+    fn image_default_size() {
+        let image = Image::default();
+        assert_eq!(Vec2::new(1.0, 1.0), image.size());
+    }
+}