### 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
#include "core/os/time.h"
#include "core/register_core_types.h"
#include "core/string/translation.h"
#include "core/version.h"
#include "drivers/register_driver_types.h"
#include "main/app_icon.gen.h"
#include "main/main_timer_sync.h"
#include "main/performance.h"
#include "main/splash.gen.h"
#include "modules/register_module_types.h"
#include "platform/register_platform_apis.h"
#include "scene/main/scene_tree.h"
#include "scene/main/window.h"
#include "scene/register_scene_types.h"
#include "scene/resources/packed_scene.h"
#include "scene/theme/theme_db.h"
#include "servers/audio_server.h"
#include "servers/camera_server.h"
#include "servers/display_server.h"
#include "servers/movie_writer/movie_writer.h"
#include "servers/movie_writer/movie_writer_mjpeg.h"
#include "servers/navigation_server_2d.h"
#include "servers/navigation_server_3d.h"
#include "servers/navigation_server_3d_dummy.h"
#include "servers/physics_server_2d.h"
#include "servers/physics_server_3d.h"
#include "servers/register_server_types.h"
#include "servers/rendering/rendering_server_default.h"
#include "servers/text/text_server_dummy.h"
#include "servers/text_server.h"