### 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 (dump_extension_api) {
		GDExtensionAPIDump::generate_extension_json_file("extension_api.json");
	}
	if (dump_gdextension_interface || dump_extension_api) {
		OS::get_singleton()->set_exit_code(EXIT_SUCCESS);
		return false;
	}
	if (validate_extension_api) {
		bool valid = GDExtensionAPIDump::validate_extension_json_file(validate_extension_api_file) == OK;
		OS::get_singleton()->set_exit_code(valid ? EXIT_SUCCESS : EXIT_FAILURE);
		return false;
	}
#ifndef DISABLE_DEPRECATED
	if (converting_project) {
		int ret = ProjectConverter3To4(converter_max_kb_file, converter_max_line_length).convert();
		if (ret) {
			OS::get_singleton()->set_exit_code(EXIT_SUCCESS);
		}
		return false;
	}
	if (validating_converting_project) {
		bool ret = ProjectConverter3To4(converter_max_kb_file, converter_max_line_length).validate_conversion();
		if (ret) {
			OS::get_singleton()->set_exit_code(EXIT_SUCCESS);
		}
		return false;
	}
#endif 
#endif 
	if (script.is_empty() && game_path.is_empty() && String(GLOBAL_GET("application/run/main_scene")) != "") {