### 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
if (openxr_api == nullptr) {
		return;
	} else {
		openxr_api->set_render_target_size_multiplier(multiplier);
	}
}
Size2 OpenXRInterface::get_render_target_size() {
	if (openxr_api == nullptr) {
		return Size2();
	} else {
		return openxr_api->get_recommended_target_size();
	}
}
uint32_t OpenXRInterface::get_view_count() {
	
	return 2;
}
void OpenXRInterface::_set_default_pos(Transform3D &p_transform, double p_world_scale, uint64_t p_eye) {
	p_transform = Transform3D();
	
	p_transform.origin.y = 1.5 * p_world_scale;
	
	if (p_eye == 1) {
		p_transform.origin.x = 0.03 * p_world_scale;
	} else if (p_eye == 2) {
		p_transform.origin.x = -0.03 * p_world_scale;
	}
}
Transform3D OpenXRInterface::get_camera_transform() {