### 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
ADD_SIGNAL(MethodInfo("play_area_changed", PropertyInfo(Variant::INT, "mode")));
	ClassDB::bind_method(D_METHOD("get_name"), &XRInterface::get_name);
	ClassDB::bind_method(D_METHOD("get_capabilities"), &XRInterface::get_capabilities);
	ClassDB::bind_method(D_METHOD("is_primary"), &XRInterface::is_primary);
	ClassDB::bind_method(D_METHOD("set_primary", "primary"), &XRInterface::set_primary);
	ClassDB::bind_method(D_METHOD("is_initialized"), &XRInterface::is_initialized);
	ClassDB::bind_method(D_METHOD("initialize"), &XRInterface::initialize);
	ClassDB::bind_method(D_METHOD("uninitialize"), &XRInterface::uninitialize);
	ClassDB::bind_method(D_METHOD("get_system_info"), &XRInterface::get_system_info);
	ClassDB::bind_method(D_METHOD("get_tracking_status"), &XRInterface::get_tracking_status);
	ClassDB::bind_method(D_METHOD("get_render_target_size"), &XRInterface::get_render_target_size);
	ClassDB::bind_method(D_METHOD("get_view_count"), &XRInterface::get_view_count);
	ClassDB::bind_method(D_METHOD("trigger_haptic_pulse", "action_name", "tracker_name", "frequency", "amplitude", "duration_sec", "delay_sec"), &XRInterface::trigger_haptic_pulse);
	ADD_GROUP("Interface", "interface_");
	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "interface_is_primary"), "set_primary", "is_primary");
	
	ClassDB::bind_method(D_METHOD("supports_play_area_mode", "mode"), &XRInterface::supports_play_area_mode);
	ClassDB::bind_method(D_METHOD("get_play_area_mode"), &XRInterface::get_play_area_mode);
	ClassDB::bind_method(D_METHOD("set_play_area_mode", "mode"), &XRInterface::set_play_area_mode);
	ClassDB::bind_method(D_METHOD("get_play_area"), &XRInterface::get_play_area);
	ADD_GROUP("XR", "xr_");
	ADD_PROPERTY(PropertyInfo(Variant::INT, "xr_play_area_mode", PROPERTY_HINT_ENUM, "Unknown,3DOF,Sitting,Roomscale,Stage"), "set_play_area_mode", "get_play_area_mode");
	
	ClassDB::bind_method(D_METHOD("get_anchor_detection_is_enabled"), &XRInterface::get_anchor_detection_is_enabled);
	ClassDB::bind_method(D_METHOD("set_anchor_detection_is_enabled", "enable"), &XRInterface::set_anchor_detection_is_enabled);
	ClassDB::bind_method(D_METHOD("get_camera_feed_id"), &XRInterface::get_camera_feed_id);
	ClassDB::bind_method(D_METHOD("is_passthrough_supported"), &XRInterface::is_passthrough_supported);
	ClassDB::bind_method(D_METHOD("is_passthrough_enabled"), &XRInterface::is_passthrough_enabled);
	ClassDB::bind_method(D_METHOD("start_passthrough"), &XRInterface::start_passthrough);