### 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
memdelete(theme_db);
	theme_db = nullptr;
}
#ifdef DEBUG_INIT
#define MAIN_PRINT(m_txt) print_line(m_txt)
#else
#define MAIN_PRINT(m_txt)
#endif
void Main::print_help(const char *p_binary) {
	print_line(String(VERSION_NAME) + " v" + get_full_version_string() + " - " + String(VERSION_WEBSITE));
	OS::get_singleton()->print("Free and open source software under the terms of the MIT license.\n");
	OS::get_singleton()->print("(c) 2014-present Godot Engine contributors.\n");
	OS::get_singleton()->print("(c) 2007-2014 Juan Linietsky, Ariel Manzur.\n");
	OS::get_singleton()->print("\n");
	OS::get_singleton()->print("Usage: %s [options] [path to scene or 'project.godot' file]\n", p_binary);
	OS::get_singleton()->print("\n");
	OS::get_singleton()->print("General options:\n");
	OS::get_singleton()->print("  -h, --help                        Display this help message.\n");
	OS::get_singleton()->print("  --version                         Display the version string.\n");
	OS::get_singleton()->print("  -v, --verbose                     Use verbose stdout mode.\n");
	OS::get_singleton()->print("  -q, --quiet                       Quiet mode, silences stdout messages. Errors are still displayed.\n");
	OS::get_singleton()->print("\n");
	OS::get_singleton()->print("Run options:\n");
	OS::get_singleton()->print("  --, ++                            Separator for user-provided arguments. Following arguments are not used by the engine, but can be read from `OS.get_cmdline_user_args()`.\n");
#ifdef TOOLS_ENABLED
	OS::get_singleton()->print("  -e, --editor                      Start the editor instead of running the scene.\n");
	OS::get_singleton()->print("  -p, --project-manager             Start the project manager, even if a project is auto-detected.\n");
	OS::get_singleton()->print("  --debug-server <uri>              Start the editor debug server (<protocol>://<host/IP>[:<port>], e.g. tcp://127.0.0.1:6007)\n");
#endif
	OS::get_singleton()->print("  --quit                            Quit after the first iteration.\n");