### 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_COND_V(session == XR_NULL_HANDLE, false);
	ERR_FAIL_NULL_V(graphics_extension, false);
	XrResult result;
	void *next_pointer = nullptr;
	for (OpenXRExtensionWrapper *wrapper : registered_extension_wrappers) {
		void *np = wrapper->set_swapchain_create_info_and_get_next_pointer(next_pointer);
		if (np != nullptr) {
			next_pointer = np;
		}
	}
	XrSwapchainCreateInfo swapchain_create_info = {
		XR_TYPE_SWAPCHAIN_CREATE_INFO, 
		next_pointer, 
		0, 
		p_usage_flags, 
		p_swapchain_format, 
		p_sample_count, 
		p_width, 
		p_height, 
		1, 
		p_array_size, 
		1 
	};
	XrSwapchain new_swapchain;
	result = xrCreateSwapchain(session, &swapchain_create_info, &new_swapchain);
	if (XR_FAILED(result)) {
		print_line("OpenXR: Failed to get swapchain [", get_error_string(result), "]");
		return false;
	}
	if (!graphics_extension->get_swapchain_image_data(new_swapchain, p_swapchain_format, p_width, p_height, p_sample_count, p_array_size, r_swapchain_graphics_data)) {