Replace the FORWARD_DECAL_MESH_HANDLE with a resource that gets cloned out in a hook.

[andriyDev]

Objective

• Related to "Always Strong" Asset Handles / Remove asset weak handles #19024.

Solution

• Rather than adding a required component Mesh3d with a default set to FORWARD_DECAL_MESH_HANDLE, we use a hook to change the Mesh3ds asset handle.

Testing

• decal example still works.

[JMS55]

Can we use a once cell or something instead, to keep the handle as a global variable?

[andriyDev]

Using a once cell or similar is probably gonna be quite controversial: it means you can only have one bevy instance running (though that might already be a limitation due to winit or something).