### 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 (!can_render()) {
		return false;
	}
	
	
	for (int i = 0; i < OPENXR_SWAPCHAIN_MAX; i++) {
		if (!swapchains[i].image_acquired && swapchains[i].swapchain != XR_NULL_HANDLE) {
			if (!acquire_image(swapchains[i])) {
				return false;
			}
			swapchains[i].image_acquired = true;
		}
	}
	for (OpenXRExtensionWrapper *wrapper : registered_extension_wrappers) {
		wrapper->on_pre_draw_viewport(p_render_target);
	}
	return true;
}
RID OpenXRAPI::get_color_texture() {
	if (swapchains[OPENXR_SWAPCHAIN_COLOR].image_acquired) {
		return graphics_extension->get_texture(swapchains[OPENXR_SWAPCHAIN_COLOR].swapchain_graphics_data, swapchains[OPENXR_SWAPCHAIN_COLOR].image_index);
	} else {
		return RID();
	}
}
RID OpenXRAPI::get_depth_texture() {