Helpers: add ProceduralSkyMaterial

ProceduralSkyMaterial is a GPU-based sky rendered via SkyMaterial that
provides the same gradient-and-sun-disk sky model as
ProceduralSkyTextureData, but evaluated on the GPU with real-time
updates and no CPU texture baking.

It exposes the same property API as ProceduralSkyTextureData
(skyTopColor, skyCurve, skyEnergy, groundBottomColor, groundCurve,
groundEnergy, sunColor, sunLatitude, sunLongitude, sunAngleMin,
sunAngleMax, sunCurve, sunEnergy), making it a drop-in replacement:
swap backgroundMode to SceneEnvironment.SkyMaterial and assign this
type to skyMaterial instead of using a Texture with
ProceduralSkyTextureData as its textureData.

Sun direction is computed from sunLatitude/sunLongitude using the same
quaternion rotation as ProceduralSkyTextureData. IBL is enabled by
default with iblSampleCount: 256, which gives quality close to the
1024-sample prefilter used by the lightProbe path.

All examples that previously used ProceduralSkyTextureData as a
lightProbe or skybox texture are updated to use ProceduralSkyMaterial.

[ChangeLog][Helpers] Added ProceduralSkyMaterial, a GPU-based
gradient sky type with the same property API as ProceduralSkyTextureData
that can be used as a drop-in replacement via SkyMaterial.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Change-Id: I64d08958e9f8036de187fc366afccb00027b0b48
Reviewed-by: Kristoffer Skau <kristoffer.skau@qt.io>