### 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
XRServer *xr_server = XRServer::get_singleton();
	ERR_FAIL_NULL_V(xr_server, Transform3D());
	Transform3D hmd_transform;
	double world_scale = xr_server->get_world_scale();
	
	hmd_transform.basis = head_transform.basis;
	hmd_transform.origin = head_transform.origin * world_scale;
	return hmd_transform;
}
Transform3D OpenXRInterface::get_transform_for_view(uint32_t p_view, const Transform3D &p_cam_transform) {
	XRServer *xr_server = XRServer::get_singleton();
	ERR_FAIL_NULL_V(xr_server, Transform3D());
	ERR_FAIL_UNSIGNED_INDEX_V_MSG(p_view, get_view_count(), Transform3D(), "View index outside bounds.");
	Transform3D t;
	if (openxr_api && openxr_api->get_view_transform(p_view, t)) {
		
		transform_for_view[p_view] = t;
	} else {
		
		t = transform_for_view[p_view];
	}
	
	double world_scale = xr_server->get_world_scale();
	t.origin *= world_scale;
	return p_cam_transform * xr_server->get_reference_frame() * t;
}
Projection OpenXRInterface::get_projection_for_view(uint32_t p_view, double p_aspect, double p_z_near, double p_z_far) {