### 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
goto error;
			}
		} else if (I->get() == "--text-driver") {
			if (I->next()) {
				text_driver = I->next()->get();
				N = I->next()->next();
			} else {
				OS::get_singleton()->print("Missing text driver argument, aborting.\n");
				goto error;
			}
		} else if (I->get() == "--display-driver") { 
			if (I->next()) {
				display_driver = I->next()->get();
				bool found = false;
				for (int i = 0; i < DisplayServer::get_create_function_count(); i++) {
					if (display_driver == DisplayServer::get_create_function_name(i)) {
						found = true;
					}
				}
				if (!found) {
					OS::get_singleton()->print("Unknown display driver '%s', aborting.\nValid options are ",
							display_driver.utf8().get_data());
					for (int i = 0; i < DisplayServer::get_create_function_count(); i++) {
						if (i == DisplayServer::get_create_function_count() - 1) {
							OS::get_singleton()->print(" and ");
						} else if (i != 0) {
							OS::get_singleton()->print(", ");
						}
						OS::get_singleton()->print("'%s'", DisplayServer::get_create_function_name(i));
					}