Make Skybox not break rendering if its image is default or 2D. (#23689)

# Objective

- Partially addresses #23688.
- Gives advice to users who accidentally create an invalid `Skybox`.
- Prevents rendering from totally breaking in that case.

## Solution

If the image is detected to be invalid, have
`prepare_skybox_bind_groups` ignore it (and warn once).

## Testing

Manually tested that `examples/3d/skybox.rs` works if the initial skybox
has the default image.

If there’s a good way to programmatically test things like this, let me
know — I didn’t see any similar tests to emulate.

Author: kpreid

crates/bevy_core_pipeline/src/skybox/mod.rs

@@ -10,6 +10,7 @@ use bevy_ecs::{
 };
 use bevy_image::BevyDefault;
 use bevy_light::Skybox;
+use bevy_log::warn_once;
 use bevy_math::Mat4;
 use bevy_render::{
     extract_component::{ComponentUniforms, DynamicUniformIndex, UniformComponentPlugin},
@@ -232,6 +233,7 @@ fn prepare_skybox_bind_groups(
             view_uniforms.uniforms.binding(),
             skybox_uniforms.binding(),
         ) && let Some(image) = images.get(image_handle)
+            && sanity_check_skybox_image_and_warn(entity, skybox, image)
         {
             let bind_group = render_device.create_bind_group(
                 "skybox_bind_group",
@@ -252,3 +254,25 @@ fn prepare_skybox_bind_groups(
         }
     }
 }
+
+fn sanity_check_skybox_image_and_warn(entity: Entity, skybox: &Skybox, image: &GpuImage) -> bool {
+    let texture_view_dimension: Option<TextureViewDimension> = image
+        .texture_view_descriptor
+        .as_ref()
+        .and_then(|desc| desc.dimension);
+    let dimension_ok = texture_view_dimension == Some(TextureViewDimension::Cube);
+    if !dimension_ok {
+        // The texture view is not a cubemap and will fail validation if rendered.
+        // In this case, we ignore the skybox so as not to break rendering.
+        //
+        // There are other possible misconfigurations which will fail and which we do not
+        // catch here, but this is a common mistake (passing an unaltered 2D image to `Skybox`).
+        warn_once!(
+            "skybox {entity}'s image {image:?} has texture view dimension \
+                        {texture_view_dimension:?}, but it must be TextureViewDimension::Cube \
+                        to render a skybox",
+            image = skybox.image
+        );
+    }
+    dimension_ok
+}