Add a "Preview Bake" button for LightmapGI for quick preview bakes

[Calinou]

Describe the project you are working on

The Godot editor 🙂

Describe the problem or limitation you are having in your project

With godotengine/godot#44628 and in the master branch, BakedLightmap can make great-looking scenes. However, the bake time is often too long to quickly iterate on. This is especially true as your scene gets more complex.

The user can temporarily switch to lower quality settings in the BakedLightmap node, but this is cumbersome and you risk accidentally committing the new settings to version control. This can cause difficult-to-diagnose issues down the line.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

When baking lightmaps, allow using low-quality settings in a temporary manner that doesn't affect the BakedLightmap's quality properties.

This kind of feature has been available for decades in various 3D modellers and map editors such as GtkRadiant, so there's a fair amount of track record for it.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

When a BakedLightmap node is selected, show 2 buttons in the 3D viewport context menu instead of one:

• Preview Bake. This bakes using low-quality settings defined in the editor settings (or perhaps project settings, for consistency across project members).
• "Final" Bake (name is subject to change). This uses the quality settings defined in the BakedLightmap node.

Make sure to add tooltips to each button to explain the difference.

If the user has performed a preview bake, there should be some kind of watermark on the 3D viewport to inform the user when the BakedLightmap node is selected. This is to make sure people don't ship preview bakes to production 🙂
Still, saving the scene with a preview bake should be allowed so that you can run your project to preview the lightmap.

If this enhancement will not be used often, can it be worked around with a few lines of script?

This can likely be implemented in an add-on, but it's not exactly trivial.

Is there a reason why this should be core and not an add-on in the asset library?

This is about improving the user experience by decreasing iteration times, so I'd argue it's important to have this in core.