This function can be used to store a custom value in the plugin's user pointer. This value
can later be obtained by other plugins by calling the hw_GetPluginUserPointer()
function. This enables plugins to expose a public interface or custom data structures to
other plugins. Other plugins can make use of your plugin by first looking for it using
hw_GetPluginList() and, if found, accessing its interface using
hw_GetPluginUserPointer().