### 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
for (OpenXRCompositionLayerProvider *provider : composition_layer_providers) {
		XrCompositionLayerBaseHeader *layer = provider->get_composition_layer();
		if (layer) {
			layers_list.push_back(layer);
		}
	}
	XrCompositionLayerProjection projection_layer = {
		XR_TYPE_COMPOSITION_LAYER_PROJECTION, 
		nullptr, 
		layers_list.size() > 0 ? XR_COMPOSITION_LAYER_BLEND_TEXTURE_SOURCE_ALPHA_BIT | XR_COMPOSITION_LAYER_CORRECT_CHROMATIC_ABERRATION_BIT : XR_COMPOSITION_LAYER_CORRECT_CHROMATIC_ABERRATION_BIT, 
		play_space, 
		view_count, 
		projection_views, 
	};
	layers_list.push_back((const XrCompositionLayerBaseHeader *)&projection_layer);
	XrFrameEndInfo frame_end_info = {
		XR_TYPE_FRAME_END_INFO, 
		nullptr, 
		frame_state.predictedDisplayTime, 
		environment_blend_mode, 
		static_cast<uint32_t>(layers_list.size()), 
		layers_list.ptr() 
	};
	result = xrEndFrame(session, &frame_end_info);
	if (XR_FAILED(result)) {
		print_line("OpenXR: failed to end frame! [", get_error_string(result), "]");
		return;
	}
}
float OpenXRAPI::get_display_refresh_rate() const {