Fix to splats frustum culling

[mvaligursky]

• Instancing used to turn off frustum culling of the mesh instance. The API now has additional parameter to control this. If the case a user sets up a customAabb on the render / model component, they can enable culling for the mesh instancing. Note that there is no culling per instance, but a single custom aabb is used to cull all instances.
• this is used by gaussian splat rendering to skip rendering when splat is off-screen

Changed API:

// note the new cull parameter, which defaults to false which was the original behaviour
MeshInstance.setInstancing(vertexBuffer, cull = false)

[LeXXik]

Ah, that was a thing. I had a screen space shader for a polyline that used instanced geometry. And when the culling was on while geometry went outside the frustum, the skybox went black among other issues. I disabled culling as a temp solution, but should revisit it after this is released. Curious if adding a frustum culling per instance would be hard?

[mvaligursky]

You can do this in the meantime:

meshInstance.setInstancing(vertexBuffer);
meshInstance.cull = true;

Per instance culling is doable, but:

• I'd need to assume instancing VB contains matrices. Currently (officially) it does, but I want to extend it to other things. And people already using it to store other things in those matrices. Hardware instancing improvements #4878
• I'd need to have an internal VB to copy data to
• I'd need to cull per camera, so added cost, even though the user can often do a single cull with larger bounds for example

So I suspect we might not add per instancing culling, and leave users to do this if needed, and update their index buffer dynamically with the results.