Remove unnecessary Option wrapper from PreviousGlobalTransform in meshlet extraction

[fairhill1]

Objective

• Fixes Requiring PreviousGlobalTransform for MeshletMesh is probably wrong #18784
• Remove unnecessary Option<> wrapper in meshlet extraction for PreviousGlobalTransform
• The existing prepass system already ensures PreviousGlobalTransform is always present, making the Option wrapper misleading

Solution

• Changed extract_meshlet_mesh_entities query from Option<&PreviousGlobalTransform> to &PreviousGlobalTransform
• Updated add_instance function parameter from Option<&PreviousGlobalTransform> to &PreviousGlobalTransform
• Simplified usage from previous_transform.map(|t| t.0).unwrap_or(transform) to previous_transform.0
• Added clarifying comments to existing update_mesh_previous_global_transforms function

The initialization logic was already working correctly - the prepass system ensures PreviousGlobalTransform equals GlobalTransform on the first frame for new entities.

Testing

• Created a test example that verifies PreviousGlobalTransform is correctly initialized to match GlobalTransform on the first frame
• Ran the test and confirmed output: "SUCCESS: PreviousGlobalTransform correctly initialized to GlobalTransform"
• No compilation errors or runtime issues observed
• The existing meshlet rendering functionality continues to work as expected

Reviewers can test by:

1. Running cargo run --example test_previous_transform to verify initialization works correctly
2. Testing any existing meshlet-based examples to ensure no regressions
3. Verifying that motion vectors still work properly in meshlet rendering

[fairhill1]

the test file in examples/ should not be merged but leaving it for now for others to test during review

[atlv24]

looks good to me (assuming the test example is removed for merge)

[JMS55]

Suggested change

let previous_transform = previous_transform.0;

Imo remove this line, just use previous_transform.0 later on directly.

[JMS55]

I don't think this fix is correct.

update_mesh_previous_global_transforms runs in PreUpdate, so any MeshletMeshes spawned in Update or later will still have the incorrect PreviousGlobalTransform.