Fix SoftBody3D for double-precision builds

[mihe]

Forgive the issue-and-PR-rolled-into-one, but I figured this was pretty straight-forward.

It turns out that my previous PR (#81298) that changed the interface for PhysicsServer3DRenderingServerHandler also broke SoftBody3D for precision=double builds entirely.

The issue is that #81298 changed the memcpy that happens in PhysicsServer3DRenderingServerHandler::set_vertex to copy the incoming vertex position by sizeof(Vector3), which obviously changes depending on the precision of the build, but the vertex buffer itself is always single-precision, leading to the memcpy writing nonsense (and eventually onto the normals part of the vertex buffer) in precision=double builds, which in turn leads to the mesh "exploding" as soon as the SoftBody3D updates the rendering server with the new particle positions.

This PR fixes this by explicitly casting each vector component to float instead of doing a memcpy.

[dsnopek]

Looks good to me!

[mihe]

This would probably also be a good thing to cherry-pick to the 4.2 branch if/when merged.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 4.2.2.