### 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
EXT_PROTO_XRRESULT_FUNC3(xrGetActionStateVector2f, (XrSession), session, (const XrActionStateGetInfo *), getInfo, (XrActionStateVector2f *), state)
	EXT_PROTO_XRRESULT_FUNC3(xrGetCurrentInteractionProfile, (XrSession), session, (XrPath), topLevelUserPath, (XrInteractionProfileState *), interactionProfile)
	EXT_PROTO_XRRESULT_FUNC2(xrGetInstanceProperties, (XrInstance), instance, (XrInstanceProperties *), instanceProperties)
	EXT_PROTO_XRRESULT_FUNC3(xrGetSystem, (XrInstance), instance, (const XrSystemGetInfo *), getInfo, (XrSystemId *), systemId)
	EXT_PROTO_XRRESULT_FUNC3(xrGetSystemProperties, (XrInstance), instance, (XrSystemId), systemId, (XrSystemProperties *), properties)
	EXT_PROTO_XRRESULT_FUNC4(xrLocateSpace, (XrSpace), space, (XrSpace), baseSpace, (XrTime), time, (XrSpaceLocation *), location)
	EXT_PROTO_XRRESULT_FUNC6(xrLocateViews, (XrSession), session, (const XrViewLocateInfo *), viewLocateInfo, (XrViewState *), viewState, (uint32_t), viewCapacityInput, (uint32_t *), viewCountOutput, (XrView *), views)
	EXT_PROTO_XRRESULT_FUNC5(xrPathToString, (XrInstance), instance, (XrPath), path, (uint32_t), bufferCapacityInput, (uint32_t *), bufferCountOutput, (char *), buffer)
	EXT_PROTO_XRRESULT_FUNC2(xrPollEvent, (XrInstance), instance, (XrEventDataBuffer *), eventData)
	EXT_PROTO_XRRESULT_FUNC2(xrReleaseSwapchainImage, (XrSwapchain), swapchain, (const XrSwapchainImageReleaseInfo *), releaseInfo)
	EXT_PROTO_XRRESULT_FUNC3(xrResultToString, (XrInstance), instance, (XrResult), value, (char *), buffer)
	EXT_PROTO_XRRESULT_FUNC3(xrStringToPath, (XrInstance), instance, (const char *), pathString, (XrPath *), path)
	EXT_PROTO_XRRESULT_FUNC2(xrSuggestInteractionProfileBindings, (XrInstance), instance, (const XrInteractionProfileSuggestedBinding *), suggestedBindings)
	EXT_PROTO_XRRESULT_FUNC2(xrSyncActions, (XrSession), session, (const XrActionsSyncInfo *), syncInfo)
	EXT_PROTO_XRRESULT_FUNC3(xrWaitFrame, (XrSession), session, (const XrFrameWaitInfo *), frameWaitInfo, (XrFrameState *), frameState)
	EXT_PROTO_XRRESULT_FUNC2(xrWaitSwapchainImage, (XrSwapchain), swapchain, (const XrSwapchainImageWaitInfo *), waitInfo)
	
	bool create_instance();
	bool get_system_info();
	bool load_supported_view_configuration_types();
	bool load_supported_environmental_blend_modes();
	bool is_view_configuration_supported(XrViewConfigurationType p_configuration_type) const;
	bool load_supported_view_configuration_views(XrViewConfigurationType p_configuration_type);
	void destroy_instance();
	
	bool create_session();
	bool load_supported_reference_spaces();
	bool is_reference_space_supported(XrReferenceSpaceType p_reference_space);
	bool setup_spaces();
	bool load_supported_swapchain_formats();