### 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
String runtime_name;
	String runtime_version;
	
	XrFormFactor form_factor = XR_FORM_FACTOR_HEAD_MOUNTED_DISPLAY;
	XrViewConfigurationType view_configuration = XR_VIEW_CONFIGURATION_TYPE_PRIMARY_STEREO;
	XrReferenceSpaceType reference_space = XR_REFERENCE_SPACE_TYPE_STAGE;
	bool submit_depth_buffer = false; 
	
	XrEnvironmentBlendMode environment_blend_mode = XR_ENVIRONMENT_BLEND_MODE_OPAQUE;
	uint32_t num_supported_environment_blend_modes = 0;
	XrEnvironmentBlendMode *supported_environment_blend_modes = nullptr;
	
	XrInstance instance = XR_NULL_HANDLE;
	XrSystemId system_id = 0;
	String system_name;
	uint32_t vendor_id = 0;
	XrSystemTrackingProperties tracking_properties;
	XrSession session = XR_NULL_HANDLE;
	XrSessionState session_state = XR_SESSION_STATE_UNKNOWN;
	bool running = false;
	XrFrameState frame_state = { XR_TYPE_FRAME_STATE, NULL, 0, 0, false };
	double render_target_size_multiplier = 1.0;
	OpenXRGraphicsExtensionWrapper *graphics_extension = nullptr;
	XrSystemGraphicsProperties graphics_properties;
	uint32_t view_count = 0;
	XrViewConfigurationView *view_configuration_views = nullptr;
	XrView *views = nullptr;
	XrCompositionLayerProjectionView *projection_views = nullptr;
	XrCompositionLayerDepthInfoKHR *depth_views = nullptr; 
	enum OpenXRSwapChainTypes {