### 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
ERR_FAIL_NULL_V(openxr_api, nullptr);
	for (int i = 0; i < p_action_set->actions.size(); i++) {
		if (p_action_set->actions[i]->action_name == p_action_name) {
			
			return nullptr;
		}
	}
	Vector<RID> tracker_rids;
	for (int i = 0; i < p_trackers.size(); i++) {
		tracker_rids.push_back(p_trackers[i]->tracker_rid);
	}
	Action *action = memnew(Action);
	if (p_action_type == OpenXRAction::OPENXR_ACTION_POSE) {
		
		
		
		if (p_action_name == "default_pose") {
			action->action_name = "default";
		} else if (p_action_name == "aim_pose") {
			action->action_name = "aim";
		} else if (p_action_name == "grip_pose") {
			action->action_name = "grip";