### 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
HashMap<String, bool *> requested_extensions;
	for (OpenXRExtensionWrapper *wrapper : registered_extension_wrappers) {
		const HashMap<String, bool *> &wrapper_request_extensions = wrapper->get_requested_extensions();
		for (const KeyValue<String, bool *> &requested_extension : wrapper_request_extensions) {
			requested_extensions[requested_extension.key] = requested_extension.value;
		}
	}
	
	enabled_extensions.clear();
	for (KeyValue<String, bool *> &requested_extension : requested_extensions) {
		if (!is_extension_supported(requested_extension.key)) {
			if (requested_extension.value == nullptr) {
				
				ERR_FAIL_V_MSG(false, String("OpenXR: OpenXR Runtime does not support ") + requested_extension.key + String(" extension!"));
			} else {
				
				*requested_extension.value = false;
			}
		} else if (requested_extension.value != nullptr) {
			
			*requested_extension.value = true;
			
			enabled_extensions.push_back(requested_extension.key.ascii());
		} else {
			
			enabled_extensions.push_back(requested_extension.key.ascii());
		}
	}