### 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
print_verbose(String("OpenXR resetting invalid display period ") + rtos(frame_state.predictedDisplayPeriod));
		frame_state.predictedDisplayPeriod = 0;
	}
	for (OpenXRExtensionWrapper *wrapper : registered_extension_wrappers) {
		wrapper->on_pre_render();
	}
	
	
	
	
	
	
	
	
	XrViewLocateInfo view_locate_info = {
		XR_TYPE_VIEW_LOCATE_INFO, 
		nullptr, 
		view_configuration, 
		frame_state.predictedDisplayTime, 
		play_space 
	};
	XrViewState view_state = {
		XR_TYPE_VIEW_STATE, 
		nullptr, 
		0 
	};
	uint32_t view_count_output;
	result = xrLocateViews(session, &view_locate_info, &view_state, view_count, &view_count_output, views);
	if (XR_FAILED(result)) {