### 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
ERR_FAIL_COND_V(session == XR_NULL_HANDLE, false);
	if (supported_swapchain_formats != nullptr) {
		
		memfree(supported_swapchain_formats);
		supported_swapchain_formats = nullptr;
	}
	XrResult result = xrEnumerateSwapchainFormats(session, 0, &num_swapchain_formats, nullptr);
	if (XR_FAILED(result)) {
		print_line("OpenXR: Failed to get swapchain format count [", get_error_string(result), "]");
		return false;
	}
	supported_swapchain_formats = (int64_t *)memalloc(sizeof(int64_t) * num_swapchain_formats);
	ERR_FAIL_NULL_V(supported_swapchain_formats, false);
	result = xrEnumerateSwapchainFormats(session, num_swapchain_formats, &num_swapchain_formats, supported_swapchain_formats);
	ERR_FAIL_COND_V_MSG(XR_FAILED(result), false, "OpenXR: Failed to enumerate swapchain formats");
	for (uint32_t i = 0; i < num_swapchain_formats; i++) {
		print_verbose(String("OpenXR: Found supported swapchain format ") + get_swapchain_format_name(supported_swapchain_formats[i]));
	}
	return true;
}
bool OpenXRAPI::is_swapchain_format_supported(int64_t p_swapchain_format) {