### 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
for (uint32_t i = 0; i < view_count; i++) {
		views[i].type = XR_TYPE_VIEW;
		views[i].next = nullptr;
		projection_views[i].type = XR_TYPE_COMPOSITION_LAYER_PROJECTION_VIEW;
		projection_views[i].next = nullptr;
		projection_views[i].subImage.swapchain = swapchains[OPENXR_SWAPCHAIN_COLOR].swapchain;
		projection_views[i].subImage.imageArrayIndex = i;
		projection_views[i].subImage.imageRect.offset.x = 0;
		projection_views[i].subImage.imageRect.offset.y = 0;
		projection_views[i].subImage.imageRect.extent.width = recommended_size.width;
		projection_views[i].subImage.imageRect.extent.height = recommended_size.height;
		if (submit_depth_buffer && OpenXRCompositionLayerDepthExtension::get_singleton()->is_available() && depth_views) {
			projection_views[i].next = &depth_views[i];
			depth_views[i].type = XR_TYPE_COMPOSITION_LAYER_DEPTH_INFO_KHR;
			depth_views[i].next = nullptr;
			depth_views[i].subImage.swapchain = swapchains[OPENXR_SWAPCHAIN_DEPTH].swapchain;
			depth_views[i].subImage.imageArrayIndex = i;
			depth_views[i].subImage.imageRect.offset.x = 0;
			depth_views[i].subImage.imageRect.offset.y = 0;
			depth_views[i].subImage.imageRect.extent.width = recommended_size.width;
			depth_views[i].subImage.imageRect.extent.height = recommended_size.height;
			depth_views[i].minDepth = 0.0;
			depth_views[i].maxDepth = 1.0;
			depth_views[i].nearZ = 0.01; 
			depth_views[i].farZ = 100.0;
		}
	};
	return true;
};
void OpenXRAPI::destroy_session() {