### 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 "servers/xr_server.h"
#ifdef TESTS_ENABLED
#include "tests/test_main.h"
#endif
#ifdef TOOLS_ENABLED
#include "editor/debugger/editor_debugger_node.h"
#include "editor/doc_data_class_path.gen.h"
#include "editor/doc_tools.h"
#include "editor/editor_help.h"
#include "editor/editor_node.h"
#include "editor/editor_paths.h"
#include "editor/editor_settings.h"
#include "editor/editor_translation.h"
#include "editor/progress_dialog.h"
#include "editor/project_manager.h"
#include "editor/register_editor_types.h"
#ifndef NO_EDITOR_SPLASH
#include "main/splash_editor.gen.h"
#endif
#ifndef DISABLE_DEPRECATED
#include "editor/project_converter_3_to_4.h"
#endif 
#endif 
#include "modules/modules_enabled.gen.h" 
#if defined(MODULE_MONO_ENABLED) && defined(TOOLS_ENABLED)
#include "modules/mono/editor/bindings_generator.h"
#endif
#ifdef MODULE_GDSCRIPT_ENABLED
#include "modules/gdscript/gdscript.h"
#endif