### 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
bool is_swapchain_format_supported(int64_t p_swapchain_format);
	bool create_swapchains();
	void destroy_session();
	
	bool create_swapchain(XrSwapchainUsageFlags p_usage_flags, int64_t p_swapchain_format, uint32_t p_width, uint32_t p_height, uint32_t p_sample_count, uint32_t p_array_size, XrSwapchain &r_swapchain, void **r_swapchain_graphics_data);
	bool acquire_image(OpenXRSwapChainInfo &p_swapchain);
	bool release_image(OpenXRSwapChainInfo &p_swapchain);
	
	struct Tracker { 
		String name; 
		XrPath toplevel_path; 
		RID active_profile_rid; 
	};
	RID_Owner<Tracker, true> tracker_owner;
	RID get_tracker_rid(XrPath p_path);
	struct ActionSet { 
		String name; 
		bool is_attached; 
		XrActionSet handle; 
	};
	RID_Owner<ActionSet, true> action_set_owner;
	struct ActionTracker { 
		RID tracker_rid; 
		XrSpace space; 
		bool was_location_valid; 
	};
	struct Action { 
		RID action_set_rid; 
		String name; 
		XrActionType action_type;