### Godot version

4.1

### System information

Windows 10 - Godot 4.1.1 - Compatibility

### Issue description

When trying to use the Varjo XR3 headset (in OpenXR mode) with Godot 4 (with OpenXR and passthrough activated), the passthrough does not work, i.e. the scene is rendered on a black background instead of the camera stream.

It seems to me that are two problems in the openxr module, more precisely in the openx_api.cpp file (fixing them solves  the issue) : 

1. The XR_COMPOSITION_LAYER_BLEND_TEXTURE_SOURCE_ALPHA_BIT  is only set when the number of layers is > 1, which is fine for the Meta Quest headset but does not work for the Varjo since it only requires one layer. 
2. The environment blend mode XR_ENVIRONMENT_BLEND_MODE_ALPHA_BLEND  is only really set when the XRFbPassthrough extension is present, which is not the case with the Varjo runtime. 

I am not sure how to fix this without causing issues for other headsets.

1. Would it be an issue to always activate the XR_COMPOSITION_LAYER_BLEND_TEXTURE_SOURCE_ALPHA_BIT ? 
2. Could blend modes be activated even if they are not supported by an extension ? 





### Steps to reproduce

- Create a Godot 4 project with OpenXR activated in Compatibility mode
- Add the code to activate OpenXr and to enable passthrough : https://docs.godotengine.org/en/stable/tutorials/xr/openxr_passthrough.html#passthrough-extension
- Start the Varjo Base software and activate OpenXR
- Run the Godot project

### Minimal reproduction project

N/A
HAND_JOINT_MIDDLE_METACARPAL = 11,
		HAND_JOINT_MIDDLE_PROXIMAL = 12,
		HAND_JOINT_MIDDLE_INTERMEDIATE = 13,
		HAND_JOINT_MIDDLE_DISTAL = 14,
		HAND_JOINT_MIDDLE_TIP = 15,
		HAND_JOINT_RING_METACARPAL = 16,
		HAND_JOINT_RING_PROXIMAL = 17,
		HAND_JOINT_RING_INTERMEDIATE = 18,
		HAND_JOINT_RING_DISTAL = 19,
		HAND_JOINT_RING_TIP = 20,
		HAND_JOINT_LITTLE_METACARPAL = 21,
		HAND_JOINT_LITTLE_PROXIMAL = 22,
		HAND_JOINT_LITTLE_INTERMEDIATE = 23,
		HAND_JOINT_LITTLE_DISTAL = 24,
		HAND_JOINT_LITTLE_TIP = 25,
		HAND_JOINT_MAX = 26,
	};
	Quaternion get_hand_joint_rotation(Hand p_hand, HandJoints p_joint) const;
	Vector3 get_hand_joint_position(Hand p_hand, HandJoints p_joint) const;
	float get_hand_joint_radius(Hand p_hand, HandJoints p_joint) const;
	Vector3 get_hand_joint_linear_velocity(Hand p_hand, HandJoints p_joint) const;
	Vector3 get_hand_joint_angular_velocity(Hand p_hand, HandJoints p_joint) const;
	OpenXRInterface();
	~OpenXRInterface();
};
VARIANT_ENUM_CAST(OpenXRInterface::Hand)
VARIANT_ENUM_CAST(OpenXRInterface::HandMotionRange)
VARIANT_ENUM_CAST(OpenXRInterface::HandJoints)
#endif