### 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
ERR_FAIL_NULL_V(supported_swapchain_formats, false);
	for (uint32_t i = 0; i < num_swapchain_formats; i++) {
		if (supported_swapchain_formats[i] == p_swapchain_format) {
			return true;
		}
	}
	return false;
}
bool OpenXRAPI::create_swapchains() {
	ERR_FAIL_NULL_V(graphics_extension, false);
	ERR_FAIL_COND_V(session == XR_NULL_HANDLE, false);
	/*
		TODO: We need to improve on this, for now we're taking our old approach of creating our main swapchains and substituting
		those for the ones Godot normally creates.
		This however means we can only use swapchains for our main XR view.
		It would have been nicer if we could override the swapchain creation in Godot with ours but we have a timing issue here.
		We can't create XR swapchains until after our XR session is fully instantiated, yet Godot creates its swapchain much earlier.
		Also Godot only creates a swapchain for the main output.
		OpenXR will require us to create swapchains as the render target for additional viewports if we want to use the layer system
		to optimize text rendering and background rendering as OpenXR may choose to reuse the results for reprojection while we're
		already rendering the next frame.
		Finally an area we need to expand upon is that Foveated rendering is only enabled for the swap chain we create,
		as we render 3D content into internal buffers that are copied into the swapchain, we do now have (basic) VRS support