A stereo packet can now carry an AuroraCockpit: tracked hands and, when the
vehicle's own wheel cannot be animated, a synthetic steering wheel or
handlebar, all in metres in the seated frame. Each eye draws it inside the
scene's pass just before the first virtual-screen draw, depth-tested with
the world's own depth mapping (captured from a full-view world draw), so
the kart and track occlude the hands and the 2D layer cannot hide them.
Hands use a runtime-provided hand mesh when one is supplied and a
procedural glove otherwise.
aurora_set_stereo_scene_anchor_scaled lets the sealed frame own its world
scale: each eye's head translation is rescaled from the packet's scale to
the frame's. A non-finite cockpit is dropped with one warning; the frame
still renders.
Ported from heurazy's mario-kart-wii-VR-port (GPL-3.0-or-later).
iChris4