### 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("session_visible"));
	ADD_SIGNAL(MethodInfo("pose_recentered"));
	
	ClassDB::bind_method(D_METHOD("get_display_refresh_rate"), &OpenXRInterface::get_display_refresh_rate);
	ClassDB::bind_method(D_METHOD("set_display_refresh_rate", "refresh_rate"), &OpenXRInterface::set_display_refresh_rate);
	ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "display_refresh_rate"), "set_display_refresh_rate", "get_display_refresh_rate");
	
	ClassDB::bind_method(D_METHOD("get_render_target_size_multiplier"), &OpenXRInterface::get_render_target_size_multiplier);
	ClassDB::bind_method(D_METHOD("set_render_target_size_multiplier", "multiplier"), &OpenXRInterface::set_render_target_size_multiplier);
	ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "render_target_size_multiplier"), "set_render_target_size_multiplier", "get_render_target_size_multiplier");
	ClassDB::bind_method(D_METHOD("is_action_set_active", "name"), &OpenXRInterface::is_action_set_active);
	ClassDB::bind_method(D_METHOD("set_action_set_active", "name", "active"), &OpenXRInterface::set_action_set_active);
	ClassDB::bind_method(D_METHOD("get_action_sets"), &OpenXRInterface::get_action_sets);
	ClassDB::bind_method(D_METHOD("get_available_display_refresh_rates"), &OpenXRInterface::get_available_display_refresh_rates);
	
	ClassDB::bind_method(D_METHOD("set_motion_range", "hand", "motion_range"), &OpenXRInterface::set_motion_range);
	ClassDB::bind_method(D_METHOD("get_motion_range", "hand"), &OpenXRInterface::get_motion_range);
	ClassDB::bind_method(D_METHOD("get_hand_joint_rotation", "hand", "joint"), &OpenXRInterface::get_hand_joint_rotation);
	ClassDB::bind_method(D_METHOD("get_hand_joint_position", "hand", "joint"), &OpenXRInterface::get_hand_joint_position);
	ClassDB::bind_method(D_METHOD("get_hand_joint_radius", "hand", "joint"), &OpenXRInterface::get_hand_joint_radius);
	ClassDB::bind_method(D_METHOD("get_hand_joint_linear_velocity", "hand", "joint"), &OpenXRInterface::get_hand_joint_linear_velocity);
	ClassDB::bind_method(D_METHOD("get_hand_joint_angular_velocity", "hand", "joint"), &OpenXRInterface::get_hand_joint_angular_velocity);
	BIND_ENUM_CONSTANT(HAND_LEFT);
	BIND_ENUM_CONSTANT(HAND_RIGHT);
	BIND_ENUM_CONSTANT(HAND_MAX);
	BIND_ENUM_CONSTANT(HAND_MOTION_RANGE_UNOBSTRUCTED);
	BIND_ENUM_CONSTANT(HAND_MOTION_RANGE_CONFORM_TO_CONTROLLER);
	BIND_ENUM_CONSTANT(HAND_MOTION_RANGE_MAX);
	BIND_ENUM_CONSTANT(HAND_JOINT_PALM);
	BIND_ENUM_CONSTANT(HAND_JOINT_WRIST);