Usability methods for RenderTargets and image handles (#10736)

# Objective

In my code I use a lot of images as render targets.
I'd like some convenience methods for working with this type.

## Solution

- Allow `.into()` to construct a `RenderTarget`
- Add `.as_image()` 

---

## Changelog

### Added

- `RenderTarget` can be constructed via `.into()` on a `Handle<Image>`
- `RenderTarget` new method: `as_image`

---------

Signed-off-by: Torstein Grindvik <torstein.grindvik@muybridge.com>
Co-authored-by: Torstein Grindvik <torstein.grindvik@muybridge.com>

crates/bevy_render/src/camera/camera.rs

@@ -428,6 +428,12 @@ pub enum RenderTarget {
     TextureView(ManualTextureViewHandle),
 }
 
+impl From<Handle<Image>> for RenderTarget {
+    fn from(handle: Handle<Image>) -> Self {
+        Self::Image(handle)
+    }
+}
+
 /// Normalized version of the render target.
 ///
 /// Once we have this we shouldn't need to resolve it down anymore.
@@ -459,6 +465,16 @@ impl RenderTarget {
             RenderTarget::TextureView(id) => Some(NormalizedRenderTarget::TextureView(*id)),
         }
     }
+
+    /// Get a handle to the render target's image,
+    /// or `None` if the render target is another variant.
+    pub fn as_image(&self) -> Option<&Handle<Image>> {
+        if let Self::Image(handle) = self {
+            Some(handle)
+        } else {
+            None
+        }
+    }
 }
 
 impl NormalizedRenderTarget {

examples/3d/render_to_texture.rs

@@ -5,7 +5,6 @@ use std::f32::consts::PI;
 use bevy::{
     prelude::*,
     render::{
-        camera::RenderTarget,
         render_resource::{
             Extent3d, TextureDescriptor, TextureDimension, TextureFormat, TextureUsages,
         },
@@ -103,7 +102,7 @@ fn setup(
             camera: Camera {
                 // render before the "main pass" camera
                 order: -1,
-                target: RenderTarget::Image(image_handle.clone()),
+                target: image_handle.clone().into(),
                 clear_color: Color::WHITE.into(),
                 ..default()
             },