### 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
emit_signal(SNAME("session_focussed"));
}
void OpenXRInterface::on_state_stopping() {
	emit_signal(SNAME("session_stopping"));
}
void OpenXRInterface::on_pose_recentered() {
	emit_signal(SNAME("pose_recentered"));
}
void OpenXRInterface::set_motion_range(const Hand p_hand, const HandMotionRange p_motion_range) {
	ERR_FAIL_INDEX(p_hand, HAND_MAX);
	ERR_FAIL_INDEX(p_motion_range, HAND_MOTION_RANGE_MAX);
	OpenXRHandTrackingExtension *hand_tracking_ext = OpenXRHandTrackingExtension::get_singleton();
	if (hand_tracking_ext && hand_tracking_ext->get_active()) {
		XrHandJointsMotionRangeEXT xr_motion_range;
		switch (p_motion_range) {
			case HAND_MOTION_RANGE_UNOBSTRUCTED:
				xr_motion_range = XR_HAND_JOINTS_MOTION_RANGE_UNOBSTRUCTED_EXT;
				break;
			case HAND_MOTION_RANGE_CONFORM_TO_CONTROLLER:
				xr_motion_range = XR_HAND_JOINTS_MOTION_RANGE_CONFORMING_TO_CONTROLLER_EXT;
				break;
			default:
				
				xr_motion_range = XR_HAND_JOINTS_MOTION_RANGE_CONFORMING_TO_CONTROLLER_EXT;
				break;
		}
		hand_tracking_ext->set_motion_range(uint32_t(p_hand), xr_motion_range);
	}
}
OpenXRInterface::HandMotionRange OpenXRInterface::get_motion_range(const Hand p_hand) const {