### 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
XR_NORMAL_TRACKING,
		XR_EXCESSIVE_MOTION,
		XR_INSUFFICIENT_FEATURES,
		XR_UNKNOWN_TRACKING,
		XR_NOT_TRACKING
	};
	enum PlayAreaMode { 
		XR_PLAY_AREA_UNKNOWN, 
		XR_PLAY_AREA_3DOF, 
		XR_PLAY_AREA_SITTING, 
		XR_PLAY_AREA_ROOMSCALE, 
		XR_PLAY_AREA_STAGE, 
	};
	enum EnvironmentBlendMode {
		XR_ENV_BLEND_MODE_OPAQUE, 
		XR_ENV_BLEND_MODE_ADDITIVE, 
		XR_ENV_BLEND_MODE_ALPHA_BLEND, 
	};
protected:
	_THREAD_SAFE_CLASS_
	static void _bind_methods();
public:
	
	virtual StringName get_name() const = 0;
	virtual uint32_t get_capabilities() const = 0;
	bool is_primary();
	void set_primary(bool p_is_primary);
	virtual bool is_initialized() const = 0;