### 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
int texture_repeat = GLOBAL_GET("rendering/textures/canvas_textures/default_texture_repeat");
			sml->get_root()->set_default_canvas_item_texture_filter(
					Viewport::DefaultCanvasItemTextureFilter(texture_filter));
			sml->get_root()->set_default_canvas_item_texture_repeat(
					Viewport::DefaultCanvasItemTextureRepeat(texture_repeat));
		}
#ifdef TOOLS_ENABLED
		if (editor) {
			bool editor_embed_subwindows = EditorSettings::get_singleton()->get_setting(
					"interface/editor/single_window_mode");
			if (editor_embed_subwindows) {
				sml->get_root()->set_embedding_subwindows(true);
			}
		}
#endif
		String local_game_path;
		if (!game_path.is_empty() && !project_manager) {
			local_game_path = game_path.replace("\\", "/");
			if (!local_game_path.begins_with("res://")) {
				bool absolute =
						(local_game_path.size() > 1) && (local_game_path[0] == '/' || local_game_path[1] == ':');
				if (!absolute) {
					if (ProjectSettings::get_singleton()->is_using_datapack()) {
						local_game_path = "res://" + local_game_path;
					} else {
						int sep = local_game_path.rfind("/");
						if (sep == -1) {
							Ref<DirAccess> da = DirAccess::create(DirAccess::ACCESS_FILESYSTEM);
							local_game_path = da->get_current_dir().path_join(local_game_path);
						} else {