### 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
OPENXR_API_INIT_XR_FUNC_V(xrEndFrame);
	OPENXR_API_INIT_XR_FUNC_V(xrEndSession);
	OPENXR_API_INIT_XR_FUNC_V(xrEnumerateEnvironmentBlendModes);
	OPENXR_API_INIT_XR_FUNC_V(xrEnumerateReferenceSpaces);
	OPENXR_API_INIT_XR_FUNC_V(xrEnumerateSwapchainFormats);
	OPENXR_API_INIT_XR_FUNC_V(xrEnumerateViewConfigurations);
	OPENXR_API_INIT_XR_FUNC_V(xrEnumerateViewConfigurationViews);
	OPENXR_API_INIT_XR_FUNC_V(xrGetActionStateBoolean);
	OPENXR_API_INIT_XR_FUNC_V(xrGetActionStateFloat);
	OPENXR_API_INIT_XR_FUNC_V(xrGetActionStateVector2f);
	OPENXR_API_INIT_XR_FUNC_V(xrGetCurrentInteractionProfile);
	OPENXR_API_INIT_XR_FUNC_V(xrGetSystem);
	OPENXR_API_INIT_XR_FUNC_V(xrGetSystemProperties);
	OPENXR_API_INIT_XR_FUNC_V(xrLocateViews);
	OPENXR_API_INIT_XR_FUNC_V(xrLocateSpace);
	OPENXR_API_INIT_XR_FUNC_V(xrPathToString);
	OPENXR_API_INIT_XR_FUNC_V(xrPollEvent);
	OPENXR_API_INIT_XR_FUNC_V(xrReleaseSwapchainImage);
	OPENXR_API_INIT_XR_FUNC_V(xrResultToString);
	OPENXR_API_INIT_XR_FUNC_V(xrStringToPath);
	OPENXR_API_INIT_XR_FUNC_V(xrSuggestInteractionProfileBindings);
	OPENXR_API_INIT_XR_FUNC_V(xrSyncActions);
	OPENXR_API_INIT_XR_FUNC_V(xrWaitFrame);
	OPENXR_API_INIT_XR_FUNC_V(xrWaitSwapchainImage);
	return true;
}
XrResult OpenXRAPI::try_get_instance_proc_addr(const char *p_name, PFN_xrVoidFunction *p_addr) {
	return xrGetInstanceProcAddr(instance, p_name, p_addr);
}
XrResult OpenXRAPI::get_instance_proc_addr(const char *p_name, PFN_xrVoidFunction *p_addr) {