### 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
String display_driver = DisplayServer::get_create_function_name(display_driver_idx);
		Vector2i *window_position = nullptr;
		Vector2i position = init_custom_pos;
		if (init_use_custom_pos) {
			window_position = &position;
		}
		Color boot_bg_color = GLOBAL_DEF_BASIC("application/boot_splash/bg_color", boot_splash_bg_color);
		DisplayServer::set_early_window_clear_color_override(true, boot_bg_color);
		
		Error err;
		display_server = DisplayServer::create(display_driver_idx, rendering_driver, window_mode, window_vsync_mode, window_flags, window_position, window_size, init_screen, err);
		if (err != OK || display_server == nullptr) {
			
			
			
			for (int i = 0; i < DisplayServer::get_create_function_count() - 1; i++) {
				if (i == display_driver_idx) {
					continue; 
				}
				display_server = DisplayServer::create(i, rendering_driver, window_mode, window_vsync_mode, window_flags, window_position, window_size, init_screen, err);
				if (err == OK && display_server != nullptr) {
					break;
				}
			}
		}
		if (err != OK || display_server == nullptr) {
			ERR_PRINT("Unable to create DisplayServer, all display drivers failed.");
			return err;
		}
	}