### 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
rendering_driver = rendering_driver.to_lower();
	if (use_custom_res) {
		if (!force_res) {
			window_size.width = GLOBAL_GET("display/window/size/viewport_width");
			window_size.height = GLOBAL_GET("display/window/size/viewport_height");
			if (globals->has_setting("display/window/size/window_width_override") &&
					globals->has_setting("display/window/size/window_height_override")) {
				int desired_width = globals->get("display/window/size/window_width_override");
				if (desired_width > 0) {
					window_size.width = desired_width;
				}
				int desired_height = globals->get("display/window/size/window_height_override");
				if (desired_height > 0) {
					window_size.height = desired_height;
				}
			}
		}
		if (!bool(GLOBAL_GET("display/window/size/resizable"))) {
			window_flags |= DisplayServer::WINDOW_FLAG_RESIZE_DISABLED_BIT;
		}
		if (bool(GLOBAL_GET("display/window/size/borderless"))) {
			window_flags |= DisplayServer::WINDOW_FLAG_BORDERLESS_BIT;
		}
		if (bool(GLOBAL_GET("display/window/size/always_on_top"))) {
			window_flags |= DisplayServer::WINDOW_FLAG_ALWAYS_ON_TOP_BIT;
		}
		if (bool(GLOBAL_GET("display/window/size/transparent"))) {
			window_flags |= DisplayServer::WINDOW_FLAG_TRANSPARENT_BIT;
		}