### 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
sml->set_debug_collisions_hint(true);
		}
		if (debug_paths) {
			sml->set_debug_paths_hint(true);
		}
		if (debug_navigation) {
			sml->set_debug_navigation_hint(true);
			NavigationServer3D::get_singleton()->set_debug_navigation_enabled(true);
		}
		if (debug_avoidance) {
			NavigationServer3D::get_singleton()->set_debug_avoidance_enabled(true);
		}
		if (debug_navigation || debug_avoidance) {
			NavigationServer3D::get_singleton()->set_active(true);
			NavigationServer3D::get_singleton()->set_debug_enabled(true);
		}
#endif
		if (single_threaded_scene) {
			sml->set_disable_node_threading(true);
		}
		bool embed_subwindows = GLOBAL_GET("display/window/subwindows/embed_subwindows");
		if (single_window || (!project_manager && !editor && embed_subwindows) || !DisplayServer::get_singleton()->has_feature(DisplayServer::Feature::FEATURE_SUBWINDOWS)) {
			sml->get_root()->set_embedding_subwindows(true);
		}
		ResourceLoader::add_custom_loaders();
		ResourceSaver::add_custom_savers();
		if (!project_manager && !editor) { /
			if (!game_path.is_empty() || !script.is_empty()) {
				/
				OS::get_singleton()->benchmark_begin_measure("load_autoloads");