### 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->get_root()->add_child(editor_node);
			if (!_export_preset.is_empty()) {
				editor_node->export_preset(_export_preset, positional_arg, export_debug, export_pack_only);
				game_path = ""; /
			}
			OS::get_singleton()->benchmark_end_measure("editor");
		}
#endif
		sml->set_auto_accept_quit(GLOBAL_GET("application/config/auto_accept_quit"));
		sml->set_quit_on_go_back(GLOBAL_GET("application/config/quit_on_go_back"));
		if (!editor && !project_manager) {
			/
			String stretch_mode = GLOBAL_GET("display/window/stretch/mode");
			String stretch_aspect = GLOBAL_GET("display/window/stretch/aspect");
			Size2i stretch_size = Size2i(GLOBAL_GET("display/window/size/viewport_width"),
					GLOBAL_GET("display/window/size/viewport_height"));
			real_t stretch_scale = GLOBAL_GET("display/window/stretch/scale");
			String stretch_scale_mode = GLOBAL_GET("display/window/stretch/scale_mode");
			Window::ContentScaleMode cs_sm = Window::CONTENT_SCALE_MODE_DISABLED;
			if (stretch_mode == "canvas_items") {
				cs_sm = Window::CONTENT_SCALE_MODE_CANVAS_ITEMS;
			} else if (stretch_mode == "viewport") {
				cs_sm = Window::CONTENT_SCALE_MODE_VIEWPORT;
			}
			Window::ContentScaleAspect cs_aspect = Window::CONTENT_SCALE_ASPECT_IGNORE;
			if (stretch_aspect == "keep") {
				cs_aspect = Window::CONTENT_SCALE_ASPECT_KEEP;
			} else if (stretch_aspect == "keep_width") {
				cs_aspect = Window::CONTENT_SCALE_ASPECT_KEEP_WIDTH;
			} else if (stretch_aspect == "keep_height") {