### 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_COND_V(instance == XR_NULL_HANDLE, RID());
	Action action;
	action.name = p_name;
	ActionSet *action_set = action_set_owner.get_or_null(p_action_set);
	ERR_FAIL_NULL_V(action_set, RID());
	ERR_FAIL_COND_V(action_set->handle == XR_NULL_HANDLE, RID());
	action.action_set_rid = p_action_set;
	switch (p_action_type) {
		case OpenXRAction::OPENXR_ACTION_BOOL:
			action.action_type = XR_ACTION_TYPE_BOOLEAN_INPUT;
			break;
		case OpenXRAction::OPENXR_ACTION_FLOAT:
			action.action_type = XR_ACTION_TYPE_FLOAT_INPUT;
			break;
		case OpenXRAction::OPENXR_ACTION_VECTOR2:
			action.action_type = XR_ACTION_TYPE_VECTOR2F_INPUT;
			break;
		case OpenXRAction::OPENXR_ACTION_POSE:
			action.action_type = XR_ACTION_TYPE_POSE_INPUT;
			break;
		case OpenXRAction::OPENXR_ACTION_HAPTIC:
			action.action_type = XR_ACTION_TYPE_VIBRATION_OUTPUT;
			break;
		default:
			ERR_FAIL_V(RID());
			break;
	}
	Vector<XrPath> toplevel_paths;
	for (int i = 0; i < p_trackers.size(); i++) {