### 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
class OpenXRInterface;
class OpenXRAPI {
private:
	
	static OpenXRAPI *singleton;
	
	static Vector<OpenXRExtensionWrapper *> registered_extension_wrappers;
	
	OpenXRInterface *xr_interface = nullptr;
	
	uint32_t num_layer_properties = 0;
	XrApiLayerProperties *layer_properties = nullptr;
	
	uint32_t num_supported_extensions = 0;
	XrExtensionProperties *supported_extensions = nullptr;
	Vector<CharString> enabled_extensions;
	
	Vector<OpenXRCompositionLayerProvider *> composition_layer_providers;
	
	uint32_t num_view_configuration_types = 0;
	XrViewConfigurationType *supported_view_configuration_types = nullptr;
	
	uint32_t num_reference_spaces = 0;
	XrReferenceSpaceType *supported_reference_spaces = nullptr;
	
	uint32_t num_swapchain_formats = 0;
	int64_t *supported_swapchain_formats = nullptr;