### 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
confidence = XRPose::XR_TRACKING_CONFIDENCE_HIGH;
		}
	} else {
		
		r_transform.origin = Vector3();
	}
	return confidence;
}
XRPose::TrackingConfidence OpenXRAPI::transform_from_location(const XrSpaceLocation &p_location, Transform3D &r_transform) {
	return _transform_from_location(p_location, r_transform);
}
XRPose::TrackingConfidence OpenXRAPI::transform_from_location(const XrHandJointLocationEXT &p_location, Transform3D &r_transform) {
	return _transform_from_location(p_location, r_transform);
}
void OpenXRAPI::parse_velocities(const XrSpaceVelocity &p_velocity, Vector3 &r_linear_velocity, Vector3 &r_angular_velocity) {
	if (p_velocity.velocityFlags & XR_SPACE_VELOCITY_LINEAR_VALID_BIT) {
		XrVector3f linear_velocity = p_velocity.linearVelocity;
		r_linear_velocity = Vector3(linear_velocity.x, linear_velocity.y, linear_velocity.z);
	} else {
		r_linear_velocity = Vector3();
	}
	if (p_velocity.velocityFlags & XR_SPACE_VELOCITY_ANGULAR_VALID_BIT) {
		XrVector3f angular_velocity = p_velocity.angularVelocity;
		r_angular_velocity = Vector3(angular_velocity.x, angular_velocity.y, angular_velocity.z);
	} else {
		r_angular_velocity = Vector3();
	}
}
bool OpenXRAPI::xr_result(XrResult result, const char *format, Array args) const {