### 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_reference_spaces, false);
	for (uint32_t i = 0; i < num_reference_spaces; i++) {
		if (supported_reference_spaces[i] == p_reference_space) {
			return true;
		}
	}
	return false;
}
bool OpenXRAPI::setup_spaces() {
	XrResult result;
	XrPosef identityPose = {
		{ 0.0, 0.0, 0.0, 1.0 },
		{ 0.0, 0.0, 0.0 }
	};
	ERR_FAIL_COND_V(session == XR_NULL_HANDLE, false);
	
	{
		if (!is_reference_space_supported(reference_space)) {
			print_line("OpenXR: reference space ", OpenXRUtil::get_reference_space_name(reference_space), " is not supported.");
			return false;
		}
		XrReferenceSpaceCreateInfo play_space_create_info = {
			XR_TYPE_REFERENCE_SPACE_CREATE_INFO, 
			nullptr,