### 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
if (submit_depth_buffer && OpenXRCompositionLayerDepthExtension::get_singleton()->is_available()) {
		
		Vector<int64_t> usable_swapchain_formats;
		int64_t swapchain_format_to_use = 0;
		graphics_extension->get_usable_depth_formats(usable_swapchain_formats);
		
		for (int i = 0; i < usable_swapchain_formats.size() && swapchain_format_to_use == 0; i++) {
			if (is_swapchain_format_supported(usable_swapchain_formats[i])) {
				swapchain_format_to_use = usable_swapchain_formats[i];
			}
		}
		if (swapchain_format_to_use == 0) {
			print_line("Couldn't find usable depth swap chain format, depth buffer will not be submitted.");
		} else {
			print_verbose(String("Using depth swap chain format:") + get_swapchain_format_name(swapchain_format_to_use));
			
			if (!create_swapchain(XR_SWAPCHAIN_USAGE_SAMPLED_BIT | XR_SWAPCHAIN_USAGE_DEPTH_STENCIL_ATTACHMENT_BIT, swapchain_format_to_use, recommended_size.width, recommended_size.height, view_configuration_views[0].recommendedSwapchainSampleCount, view_count, swapchains[OPENXR_SWAPCHAIN_DEPTH].swapchain, &swapchains[OPENXR_SWAPCHAIN_DEPTH].swapchain_graphics_data)) {
				return false;
			}
			depth_views = (XrCompositionLayerDepthInfoKHR *)memalloc(sizeof(XrCompositionLayerDepthInfoKHR) * view_count);
			ERR_FAIL_NULL_V_MSG(depth_views, false, "OpenXR Couldn't allocate memory for depth views");
		}
	}
	
	
	{
		
	}