### 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
String audio_driver = "";
	String project_path = ".";
	bool upwards = false;
	String debug_uri = "";
	bool skip_breakpoints = false;
	String main_pack;
	bool quiet_stdout = false;
	int rtm = -1;
	String remotefs;
	String remotefs_pass;
	Vector<String> breakpoints;
	bool use_custom_res = true;
	bool force_res = false;
	bool delta_smoothing_override = false;
	String default_renderer = "";
	String default_renderer_mobile = "";
	String renderer_hints = "";
	packed_data = PackedData::get_singleton();
	if (!packed_data) {
		packed_data = memnew(PackedData);
	}
#ifdef MINIZIP_ENABLED
	
	zip_packed_data = ZipArchive::get_singleton();
	
	if (!zip_packed_data) {
		zip_packed_data = memnew(ZipArchive);
	}
	packed_data->add_pack_source(zip_packed_data);
#endif