### 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
OS::get_singleton()->_allow_hidpi = true;
		
		OS::get_singleton()->set_environment("DISABLE_MANGOHUD", "1"); 
		OS::get_singleton()->set_environment("DISABLE_RTSS_LAYER", "1"); 
		OS::get_singleton()->set_environment("DISABLE_VKBASALT", "1");
	} else {
		
		OS::get_singleton()->unset_environment("DISABLE_MANGOHUD");
		OS::get_singleton()->unset_environment("DISABLE_RTSS_LAYER");
		OS::get_singleton()->unset_environment("DISABLE_VKBASALT");
	}
#endif
	if (rtm == -1) {
		rtm = GLOBAL_DEF("rendering/driver/threads/thread_model", OS::RENDER_THREAD_SAFE);
	}
	if (rtm >= 0 && rtm < 3) {
		if (editor || project_manager) {
			
			rtm = OS::RENDER_THREAD_SAFE;
		}
		OS::get_singleton()->_render_thread_mode = OS::RenderThreadMode(rtm);
	}
	
	
	
	DEV_ASSERT(NULL_DISPLAY_DRIVER == DisplayServer::get_create_function_name(DisplayServer::get_create_function_count() - 1));
	for (int i = 0; i < DisplayServer::get_create_function_count(); i++) {
		String name = DisplayServer::get_create_function_name(i);
		if (display_driver == name) {