### 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
ClassDB::bind_method(D_METHOD("stop_passthrough"), &XRInterface::stop_passthrough);
	ClassDB::bind_method(D_METHOD("get_transform_for_view", "view", "cam_transform"), &XRInterface::get_transform_for_view);
	ClassDB::bind_method(D_METHOD("get_projection_for_view", "view", "aspect", "near", "far"), &XRInterface::get_projection_for_view);
	
	ClassDB::bind_method(D_METHOD("get_supported_environment_blend_modes"), &XRInterface::get_supported_environment_blend_modes);
	ClassDB::bind_method(D_METHOD("set_environment_blend_mode", "mode"), &XRInterface::set_environment_blend_mode);
	ADD_GROUP("AR", "ar_");
	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "ar_is_anchor_detection_enabled"), "set_anchor_detection_is_enabled", "get_anchor_detection_is_enabled");
	BIND_ENUM_CONSTANT(XR_NONE);
	BIND_ENUM_CONSTANT(XR_MONO);
	BIND_ENUM_CONSTANT(XR_STEREO);
	BIND_ENUM_CONSTANT(XR_QUAD);
	BIND_ENUM_CONSTANT(XR_VR);
	BIND_ENUM_CONSTANT(XR_AR);
	BIND_ENUM_CONSTANT(XR_EXTERNAL);
	BIND_ENUM_CONSTANT(XR_NORMAL_TRACKING);
	BIND_ENUM_CONSTANT(XR_EXCESSIVE_MOTION);
	BIND_ENUM_CONSTANT(XR_INSUFFICIENT_FEATURES);
	BIND_ENUM_CONSTANT(XR_UNKNOWN_TRACKING);
	BIND_ENUM_CONSTANT(XR_NOT_TRACKING);
	BIND_ENUM_CONSTANT(XR_PLAY_AREA_UNKNOWN);
	BIND_ENUM_CONSTANT(XR_PLAY_AREA_3DOF);
	BIND_ENUM_CONSTANT(XR_PLAY_AREA_SITTING);
	BIND_ENUM_CONSTANT(XR_PLAY_AREA_ROOMSCALE);
	BIND_ENUM_CONSTANT(XR_PLAY_AREA_STAGE);
	BIND_ENUM_CONSTANT(XR_ENV_BLEND_MODE_OPAQUE);
	BIND_ENUM_CONSTANT(XR_ENV_BLEND_MODE_ADDITIVE);
	BIND_ENUM_CONSTANT(XR_ENV_BLEND_MODE_ALPHA_BLEND);
};
bool XRInterface::is_primary() {