Lightmap baking blocks

Baking a scene is currently a blocking operation, rendering the application unusable until the process completes. When baking via DebugView, this results in a frozen application, and when baking via the CLI, a white window appears that never completes rendering before the application exits.

This behavior occurs because the lightmap baker uses the main renderer for GPU tasks during the baking process, interleaved with heavy CPU work.

A potential solution is to implement a dedicated off-screen renderer specifically for the lightmapper. This would allow the baking process to run independently, with a single callback notifying the application when baking is complete.

This change would enable the application to remain responsive during baking. With some additional work, it could also improve the CLI experience by avoiding the need to show a window at all.