### 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
#endif
	EXT_PROTO_XRRESULT_FUNC3(xrAcquireSwapchainImage, (XrSwapchain), swapchain, (const XrSwapchainImageAcquireInfo *), acquireInfo, (uint32_t *), index)
	EXT_PROTO_XRRESULT_FUNC3(xrApplyHapticFeedback, (XrSession), session, (const XrHapticActionInfo *), hapticActionInfo, (const XrHapticBaseHeader *), hapticFeedback)
	EXT_PROTO_XRRESULT_FUNC2(xrAttachSessionActionSets, (XrSession), session, (const XrSessionActionSetsAttachInfo *), attachInfo)
	EXT_PROTO_XRRESULT_FUNC2(xrBeginFrame, (XrSession), session, (const XrFrameBeginInfo *), frameBeginInfo)
	EXT_PROTO_XRRESULT_FUNC2(xrBeginSession, (XrSession), session, (const XrSessionBeginInfo *), beginInfo)
	EXT_PROTO_XRRESULT_FUNC3(xrCreateAction, (XrActionSet), actionSet, (const XrActionCreateInfo *), createInfo, (XrAction *), action)
	EXT_PROTO_XRRESULT_FUNC3(xrCreateActionSet, (XrInstance), instance, (const XrActionSetCreateInfo *), createInfo, (XrActionSet *), actionSet)
	EXT_PROTO_XRRESULT_FUNC3(xrCreateActionSpace, (XrSession), session, (const XrActionSpaceCreateInfo *), createInfo, (XrSpace *), space)
	EXT_PROTO_XRRESULT_FUNC2(xrCreateInstance, (const XrInstanceCreateInfo *), createInfo, (XrInstance *), instance)
	EXT_PROTO_XRRESULT_FUNC3(xrCreateReferenceSpace, (XrSession), session, (const XrReferenceSpaceCreateInfo *), createInfo, (XrSpace *), space)
	EXT_PROTO_XRRESULT_FUNC3(xrCreateSession, (XrInstance), instance, (const XrSessionCreateInfo *), createInfo, (XrSession *), session)
	EXT_PROTO_XRRESULT_FUNC3(xrCreateSwapchain, (XrSession), session, (const XrSwapchainCreateInfo *), createInfo, (XrSwapchain *), swapchain)
	EXT_PROTO_XRRESULT_FUNC1(xrDestroyAction, (XrAction), action)
	EXT_PROTO_XRRESULT_FUNC1(xrDestroyActionSet, (XrActionSet), actionSet)
	EXT_PROTO_XRRESULT_FUNC1(xrDestroyInstance, (XrInstance), instance)
	EXT_PROTO_XRRESULT_FUNC1(xrDestroySession, (XrSession), session)
	EXT_PROTO_XRRESULT_FUNC1(xrDestroySpace, (XrSpace), space)
	EXT_PROTO_XRRESULT_FUNC1(xrDestroySwapchain, (XrSwapchain), swapchain)
	EXT_PROTO_XRRESULT_FUNC2(xrEndFrame, (XrSession), session, (const XrFrameEndInfo *), frameEndInfo)
	EXT_PROTO_XRRESULT_FUNC1(xrEndSession, (XrSession), session)
	EXT_PROTO_XRRESULT_FUNC3(xrEnumerateApiLayerProperties, (uint32_t), propertyCapacityInput, (uint32_t *), propertyCountOutput, (XrApiLayerProperties *), properties)
	EXT_PROTO_XRRESULT_FUNC6(xrEnumerateEnvironmentBlendModes, (XrInstance), instance, (XrSystemId), systemId, (XrViewConfigurationType), viewConfigurationType, (uint32_t), environmentBlendModeCapacityInput, (uint32_t *), environmentBlendModeCountOutput, (XrEnvironmentBlendMode *), environmentBlendModes)
	EXT_PROTO_XRRESULT_FUNC4(xrEnumerateInstanceExtensionProperties, (const char *), layerName, (uint32_t), propertyCapacityInput, (uint32_t *), propertyCountOutput, (XrExtensionProperties *), properties)
	EXT_PROTO_XRRESULT_FUNC4(xrEnumerateReferenceSpaces, (XrSession), session, (uint32_t), spaceCapacityInput, (uint32_t *), spaceCountOutput, (XrReferenceSpaceType *), spaces)
	EXT_PROTO_XRRESULT_FUNC4(xrEnumerateSwapchainFormats, (XrSession), session, (uint32_t), formatCapacityInput, (uint32_t *), formatCountOutput, (int64_t *), formats)
	EXT_PROTO_XRRESULT_FUNC5(xrEnumerateViewConfigurations, (XrInstance), instance, (XrSystemId), systemId, (uint32_t), viewConfigurationTypeCapacityInput, (uint32_t *), viewConfigurationTypeCountOutput, (XrViewConfigurationType *), viewConfigurationTypes)
	EXT_PROTO_XRRESULT_FUNC6(xrEnumerateViewConfigurationViews, (XrInstance), instance, (XrSystemId), systemId, (XrViewConfigurationType), viewConfigurationType, (uint32_t), viewCapacityInput, (uint32_t *), viewCountOutput, (XrViewConfigurationView *), views)
	EXT_PROTO_XRRESULT_FUNC3(xrGetActionStateBoolean, (XrSession), session, (const XrActionStateGetInfo *), getInfo, (XrActionStateBoolean *), state)
	EXT_PROTO_XRRESULT_FUNC3(xrGetActionStateFloat, (XrSession), session, (const XrActionStateGetInfo *), getInfo, (XrActionStateFloat *), state)