### 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("Unknown renderer name '%s', aborting. Valid options are: %s\n", rendering_method.utf8().get_data(), renderer_hints.utf8().get_data());
			goto error;
		}
	}
	if (!rendering_driver.is_empty()) {
		
		
		
		bool found = false;
		for (int i = 0; i < DisplayServer::get_create_function_count(); i++) {
			Vector<String> r_drivers = DisplayServer::get_create_function_rendering_drivers(i);
			for (int d = 0; d < r_drivers.size(); d++) {
				if (rendering_driver == r_drivers[d]) {
					found = true;
					break;
				}
			}
		}
		if (!found) {
			OS::get_singleton()->print("Unknown rendering driver '%s', aborting.\nValid options are ",
					rendering_driver.utf8().get_data());
			for (int i = 0; i < DisplayServer::get_create_function_count(); i++) {
				Vector<String> r_drivers = DisplayServer::get_create_function_rendering_drivers(i);
				for (int d = 0; d < r_drivers.size(); d++) {
					OS::get_singleton()->print("'%s', ", r_drivers[d].utf8().get_data());
				}
			}
			OS::get_singleton()->print(".\n");
			goto error;
		}