Revert "[3D] Add gradient background to 3D scene"

[wonder-sk]

Reverts #65820 per Nyall's request

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[nyalldawson]

@wonder-sk Sorry for the hassle - let's avoid the revert here, I think we can do the cleanups as follow ups.

My outstanding issues are:

1. From an API perspective, I would really like to see the background settings exposed as a subclass of QgsSkyboxSettings (or just settings within that class, since it's a messy class anyway at the moment!). In https://github.com/nyalldawson/QGIS/tree/asset_3d I'm moving toward replacing Material settings with more generic "3d assets" in the style library, in order to allow us to store skybox setups in the style library. And it would be very nice if this also allowed storage of the background gradient settings too!
2. I would really love us to use QgsGradientColorRamp here instead of the limiting 2-color gradient approach. We have very nice widgets for configuring gradient ramps, and I can definitely see a use case for more than 2 color background gradients (eg fake sunsets)

[nirvn]

Big +1 on point nb 2 :)

[uclaros]

Please keep in mind that the gradient background is not a skybox. It's supposed to be just a background to make working with the 3d view more comfortable. It does not follow the camera and should not be used to stylize the 3d view for presentation, that's what the skybox is for.

My take would be to keep the gradient background as a simple 2 color gradient and move its setting to a project wide setting. To me it is a slight extension to the existing background color project setting and should be treated the same. So no per 3d view configuration of colors. In the 3d view's settings one would only pick whether he needs a skybox rendered on top or not. Bonus is we keep the project xml tidier - less items in the 3d views' xml.

For stylizing a 3d scene we should guide users into using the skybox instead. I know finding and tweaking a sunset skybox is much more complicated than picking a few colors and ramp, so instead of "abusing" the background gradient for that, we could add a "dynamic skybox" option.
User would define a QgsGradientColorRamp and the skybox image would be generated on the fly for it.

[ViperMiniQ]

Sorry if it hasn't been communicated clearly, but it is a static background, not a texture that is applied on the Skybox, like @nyalldawson you showed in your working branch for the color ramp approach.

Screencast_20260427_121546.webm