### 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
static bool init_windowed = false;
static bool init_always_on_top = false;
static bool init_use_custom_pos = false;
static bool init_use_custom_screen = false;
static Vector2 init_custom_pos;
static bool use_debug_profiler = false;
#ifdef DEBUG_ENABLED
static bool debug_collisions = false;
static bool debug_paths = false;
static bool debug_navigation = false;
static bool debug_avoidance = false;
#endif
static int max_fps = -1;
static int frame_delay = 0;
static int audio_output_latency = 0;
static bool disable_render_loop = false;
static int fixed_fps = -1;
static MovieWriter *movie_writer = nullptr;
static bool disable_vsync = false;
static bool print_fps = false;
#ifdef TOOLS_ENABLED
static bool dump_gdextension_interface = false;
static bool dump_extension_api = false;
static bool validate_extension_api = false;
static String validate_extension_api_file;
#endif
bool profile_gpu = false;
static const String NULL_DISPLAY_DRIVER("headless");
static const String NULL_AUDIO_DRIVER("Dummy");
bool Main::is_cmdline_tool() {