### 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
if (bool(GLOBAL_GET("display/window/size/extend_to_title"))) {
			window_flags |= DisplayServer::WINDOW_FLAG_EXTEND_TO_TITLE_BIT;
		}
		if (bool(GLOBAL_GET("display/window/size/no_focus"))) {
			window_flags |= DisplayServer::WINDOW_FLAG_NO_FOCUS_BIT;
		}
		window_mode = (DisplayServer::WindowMode)(GLOBAL_GET("display/window/size/mode").operator int());
		int initial_position_type = GLOBAL_GET("display/window/size/initial_position_type").operator int();
		if (initial_position_type == 0) {
			if (!init_use_custom_pos) {
				init_custom_pos = GLOBAL_GET("display/window/size/initial_position").operator Vector2i();
				init_use_custom_pos = true;
			}
		} else if (initial_position_type == 1) {
			if (!init_use_custom_screen) {
				init_screen = DisplayServer::SCREEN_PRIMARY;
				init_use_custom_screen = true;
			}
		} else if (initial_position_type == 2) {
			if (!init_use_custom_screen) {
				init_screen = GLOBAL_GET("display/window/size/initial_screen").operator int();
				init_use_custom_screen = true;
			}
		}
	}
	GLOBAL_DEF("internationalization/locale/include_text_server_data", false);
	OS::get_singleton()->_allow_hidpi = GLOBAL_DEF("display/window/dpi/allow_hidpi", true);
	OS::get_singleton()->_allow_layered = GLOBAL_DEF("display/window/per_pixel_transparency/allowed", false);
#ifdef TOOLS_ENABLED
	if (editor || project_manager) {