### 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
OS::get_singleton()->print("  --quit-after <int>                Quit after the given number of iterations. Set to 0 to disable.\n");
	OS::get_singleton()->print("  -l, --language <locale>           Use a specific locale (<locale> being a two-letter code).\n");
	OS::get_singleton()->print("  --path <directory>                Path to a project (<directory> must contain a 'project.godot' file).\n");
	OS::get_singleton()->print("  -u, --upwards                     Scan folders upwards for project.godot file.\n");
	OS::get_singleton()->print("  --main-pack <file>                Path to a pack (.pck) file to load.\n");
	OS::get_singleton()->print("  --render-thread <mode>            Render thread mode ['unsafe', 'safe', 'separate'].\n");
	OS::get_singleton()->print("  --remote-fs <address>             Remote filesystem (<host/IP>[:<port>] address).\n");
	OS::get_singleton()->print("  --remote-fs-password <password>   Password for remote filesystem.\n");
	OS::get_singleton()->print("  --audio-driver <driver>           Audio driver [");
	for (int i = 0; i < AudioDriverManager::get_driver_count(); i++) {
		if (i > 0) {
			OS::get_singleton()->print(", ");
		}
		OS::get_singleton()->print("'%s'", AudioDriverManager::get_driver(i)->get_name());
	}
	OS::get_singleton()->print("].\n");
	OS::get_singleton()->print("  --display-driver <driver>         Display driver (and rendering driver) [");
	for (int i = 0; i < DisplayServer::get_create_function_count(); i++) {
		if (i > 0) {
			OS::get_singleton()->print(", ");
		}
		OS::get_singleton()->print("'%s' (", DisplayServer::get_create_function_name(i));
		Vector<String> rd = DisplayServer::get_create_function_rendering_drivers(i);
		for (int j = 0; j < rd.size(); j++) {
			if (j > 0) {
				OS::get_singleton()->print(", ");
			}
			OS::get_singleton()->print("'%s'", rd[j].utf8().get_data());
		}
		OS::get_singleton()->print(")");