Add the VR cockpit overlay to Aurora's eye replay

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).

aurora-main/lib/gfx/common.hpp

@@ -314,6 +314,8 @@ struct StereoReplayEye {
 
 struct StereoReplayFrame {
   std::array<StereoReplayEye, AURORA_STEREO_EYE_COUNT> eyes;
+  // VR hands and synthetic wheel, drawn per eye after the world (gfx/cockpit.hpp).
+  AuroraCockpit cockpit{};
 };
 
 void end_frame(const wgpu::CommandEncoder& cmd);