Resolve ambiguity around RPC glTF importing

[jwnimmer-tri]

We apply a 90 degree rotation after importing the glTF we got over RPC. It is not sufficiently clear why we do this.

Is Blender importing the file incorrectly? Or is its importer documentation unclear about applying extra rotations that we need to undo? Or is Drake exporting the scene incorrectly? It has to be one or those, so we need figure out which one and (at least) write it down. If it's Drake doing it wrong, we'll also need to fix that.

Sean's comment from review:

        bpy.ops.transform.rotate(
            value=math.pi/2, orient_axis='X', orient_type='GLOBAL'
        )

Does this feel like a defect in VTK's glTF exporter? glTF is documented as y-up. Blender is z-up. If we need to rotate positive 90 degrees around the x-axis, it's because blender applied, correctly, the negative 90 degree rotation to go back from y-up to z-up. Admittedly, as long as the server knows about the idiosyncracies of the client's glTF flavor, they can form a coherent whole. But it does mean we're producing non-compliant glTF files.

[jwnimmer-tri]

BTW See also some transform rejiggering in https://github.com/RobotLocomotion/drake/blob/d602cf9109f4d1f2d9f114bdd24b1b32290adfdc/geometry/render_gltf_client/internal_render_engine_gltf_client.cc#L186 in case its relevant.