feat: route stereo video through eye-specific surfaces

[juelg]

Summary

• replace the stereo eye-mask RawImage path in VideoStreamReceiver
• render left and right video frames on dedicated eye-specific quad surfaces instead
• keep the existing mono path for non-stereo streams

Why

The current RawImage plus custom unity_StereoEyeIndex shader path is too brittle on Quest and is the likely reason both stereo views end up alternating or leaking into both eyes. This change moves the actual stereo imagery onto dedicated left/right renderers that can be culled by eye-specific cameras.

What changed

• keep mono video on the existing double-buffered RawImage path
• for stereo video, create left/right quad surfaces as children of the video window
• assign those surfaces to IRISStereoLeft and IRISStereoRight
• use unlit textured materials instead of the eye-mask shader path for stereo presentation

Notes

• This PR depends on the companion Quest scene/camera PR.
• The Quest repo should pin IRIS-Viz commit f59927ca446f0b58ffdc78539ae873fb082ec561 in Packages/packages-lock.json.