Add support for MultiMesh GroundedSpatial Assets for better performance

[kb173]

There are a lot of individual assets and even more individual meshes when having multiple large PV plants active. This causes a noticeable performance drop.

We should omptimize the model (fewer individual meshes, ideally just 1 material) and maybe use a multimesh for the fields.

[kb173]

6f653ed made a big improvement, but the model could still be simplified: e.g. individual 3D panels aren't necessary, we could have one big flat mesh with a repeating texture (and perhaps switch to the more detailed model when the player is close?)

[kb173]

Around 5 nearby large PV fields seem to work well performance-wise. I think the bottleneck is actually the high amount of Nodes, not the meshes.

Let's see on Monday whether we need to investigate this more and improve performance, or whether we won't have that many PV fields on one place anyways. If we do need to improve it, turning it into a multimesh should work.

[kb173]

We placed assets pretty densely on Monday and the performance didn't seem to be a problem - seems like reducing the nodes (6f653ed) was enough.

Generalizing this issue for maybe in the future