### 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
audio_driver_idx = AudioDriverManager::get_driver_count() - 1;
		AudioDriverDummy::get_dummy_singleton()->set_use_threads(false);
	}
	{
		window_orientation = DisplayServer::ScreenOrientation(int(GLOBAL_DEF_BASIC("display/window/handheld/orientation", DisplayServer::ScreenOrientation::SCREEN_LANDSCAPE)));
	}
	{
		window_vsync_mode = DisplayServer::VSyncMode(int(GLOBAL_DEF_BASIC("display/window/vsync/vsync_mode", DisplayServer::VSyncMode::VSYNC_ENABLED)));
		if (disable_vsync) {
			window_vsync_mode = DisplayServer::VSyncMode::VSYNC_DISABLED;
		}
	}
	Engine::get_singleton()->set_physics_ticks_per_second(GLOBAL_DEF_BASIC(PropertyInfo(Variant::INT, "physics/common/physics_ticks_per_second", PROPERTY_HINT_RANGE, "1,1000,1"), 60));
	Engine::get_singleton()->set_max_physics_steps_per_frame(GLOBAL_DEF_BASIC(PropertyInfo(Variant::INT, "physics/common/max_physics_steps_per_frame", PROPERTY_HINT_RANGE, "1,100,1"), 8));
	Engine::get_singleton()->set_physics_jitter_fix(GLOBAL_DEF("physics/common/physics_jitter_fix", 0.5));
	Engine::get_singleton()->set_max_fps(GLOBAL_DEF(PropertyInfo(Variant::INT, "application/run/max_fps", PROPERTY_HINT_RANGE, "0,1000,1"), 0));
	Engine::get_singleton()->set_audio_output_latency(GLOBAL_DEF_RST(PropertyInfo(Variant::INT, "audio/driver/output_latency", PROPERTY_HINT_RANGE, "1,100,1"), 15));
	
	GLOBAL_DEF_RST("audio/driver/output_latency.web", 50);
	GLOBAL_DEF("debug/settings/stdout/print_fps", false);
	GLOBAL_DEF("debug/settings/stdout/print_gpu_profile", false);
	GLOBAL_DEF("debug/settings/stdout/verbose_stdout", false);
	if (!OS::get_singleton()->_verbose_stdout) { 
		OS::get_singleton()->_verbose_stdout = GLOBAL_GET("debug/settings/stdout/verbose_stdout");
	}
#if defined(MACOS_ENABLED) || defined(IOS_ENABLED)
	OS::get_singleton()->set_environment("MVK_CONFIG_LOG_LEVEL", OS::get_singleton()->_verbose_stdout ? "3" : "1"); 
#endif
	if (max_fps >= 0) {