### 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
Vector<BlitToScreen> blit_to_screen;
#ifndef ANDROID_ENABLED
	
	if (p_screen_rect != Rect2()) {
		BlitToScreen blit;
		blit.render_target = p_render_target;
		blit.multi_view.use_layer = true;
		blit.multi_view.layer = 0;
		blit.lens_distortion.apply = false;
		Size2 render_size = get_render_target_size();
		Rect2 dst_rect = p_screen_rect;
		float new_height = dst_rect.size.x * (render_size.y / render_size.x);
		if (new_height > dst_rect.size.y) {
			dst_rect.position.y = (0.5 * dst_rect.size.y) - (0.5 * new_height);
			dst_rect.size.y = new_height;
		} else {
			float new_width = dst_rect.size.y * (render_size.x / render_size.y);
			dst_rect.position.x = (0.5 * dst_rect.size.x) - (0.5 * new_width);
			dst_rect.size.x = new_width;
		}
		blit.dst_rect = dst_rect;
		blit_to_screen.push_back(blit);
	}
#endif
	if (openxr_api) {
		openxr_api->post_draw_viewport(p_render_target);
	}
	return blit_to_screen;
}
void OpenXRInterface::end_frame() {