Refactor affects_lightmapped_meshes, introduce AmbientLightFlags

[coreh]

Objective

• Make ambient light more consistent with other light sources in the code
• Pave the way for more ambient light boolean flags other than affects_lightmapped_meshes
• Make it easier to support monochromatic ambient lights as a follow up once Minimum Viable Monochromatic Lights #24428 lands

Solution

• Add AmbientLightFlags bitflags struct
• Rename ambient_light_affects_lightmapped_meshes to ambient_light_flags in GpuLights struct and matching WGSL-side Lights struct
• Added WGSL AMBIENT_LIGHT_FLAGS_AFFECTS_LIGHTMAPPED_MESHES_BIT constant
• Set up plumbing to both pass and use the new bitfield consistently with other light flags

Testing

• Did you test these changes? If so, how? Yes, by viewing the lighting and lightmaps examples
• Are there any parts that need more testing? Not that I know of
• How can other people (reviewers) test your changes? Is there anything specific they need to know? Manually verify ambient light still works
• If relevant, what platforms did you test these changes on, and are there any important ones you can't test? macOS

[olukowski]

Code looks good, and ambient light still works properly in the lighting example on my machine.

[alice-i-cecile]

Yeah, this makes sense as a cleanup to me.

[coreh]

One heads up is that @Elabajaba mentioned that ambient light will be replaced, so this might be short-lived:

https://discord.com/channels/691052431525675048/743663924229963868/1510811921782280282

[alice-i-cecile]

Yep, that's my hope too. AmbientLight is rather a trap currently, and produced a very dated look. Useful for some very specific retro projects, but not something we should be shipping.

[alice-i-cecile]

#18207 is the breadcrumb to the "remove ambient light" work.