### 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
uint64_t size = uint64_t(action->trackers.size());
	for (uint64_t i = 0; i < size && index == 0xFFFFFFFF; i++) {
		if (action->trackers[i].tracker_rid == p_tracker) {
			index = i;
		}
	}
	if (index == 0xFFFFFFFF) {
		
		return XRPose::XR_TRACKING_CONFIDENCE_NONE;
	}
	XrTime display_time = get_next_frame_time();
	if (display_time == 0) {
		return XRPose::XR_TRACKING_CONFIDENCE_NONE;
	}
	if (action->trackers[index].space == XR_NULL_HANDLE) {
		
		XrActionSpaceCreateInfo action_space_info = {
			XR_TYPE_ACTION_SPACE_CREATE_INFO, 
			nullptr, 
			action->handle, 
			tracker->toplevel_path, 
			{
					{ 0.0, 0.0, 0.0, 1.0 }, 
					{ 0.0, 0.0, 0.0 } 
			} 
		};
		XrSpace space;
		XrResult result = xrCreateActionSpace(session, &action_space_info, &space);
		if (XR_FAILED(result)) {
			print_line("OpenXR: couldn't create action space! [", get_error_string(result), "]");