Cache lightprobe generation in ligthtmap baking

[reduz]

Implements half of godotengine/godot-proposals#7900

[Calinou]

Tested locally, it works as expected.

Testing project: test_lightmap_preview_bake_4.x.zip

Time for a lightmap bake with Probe Subdiv set to 16 (i9-13900K + RTX 4090):

• Cold cache: 7 seconds
• Warm cache: 4 seconds

Some notes:

• Creating a LightmapProbe invalidates cache (as expected).
• Creating or removing a static object with UV2 doesn't invalidate cache (as expected, but I initially thought this wasn't feasible).
• Moving a dynamic object then baking doesn't invalidate cache (as expected).

What I find unexpected is that when you restart the editor, the cache is lost (even if you save the scene after baking). This means you only get the warm cache benefit for subsequent bakes within a given editor session. I'd expect it to be able to persist the cache across restarts.

[jcostello]

I expect to persist as well after the editor is restarted as well

[reduz]

@Calinou I fixed a bug, the cache should persist. Feel free to give it another try.

[Calinou]

Probe generation caching now works across editor restarts.

However, it only seems to work if you've baked the same scene at least twice before exiting the editor. If you only bake it once, caching will not be effective when you reopen the editor and bake again without changing anything.