Fix shared material bug: duplicate material per webcam quad instance

Godot 4 shares sub_resource materials across scene instances by default,
so all 3 camera quads were showing the same (last updated) texture.
Duplicating the material in _ready() gives each instance its own texture.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/webcam_display.gd

@@ -18,9 +18,12 @@ var _has_received_frame: bool = false
 
 
 func _ready() -> void:
-	# Get or create the material
+	# Get or create the material — duplicate to ensure each instance has its own
 	_material = material_override as StandardMaterial3D
-	if _material == null:
+	if _material != null:
+		_material = _material.duplicate() as StandardMaterial3D
+		material_override = _material
+	else:
 		_material = StandardMaterial3D.new()
 		material_override = _material