I added -Wshadow and then turned it off again because of massive variable shadowing in the blit macros.
Feel free to go through that code and fix these if you want. Just uncomment CheckWarnShadow in configure.in if you want to try this.
It was simpler to just have the polling (actually: hotplug detection)
functions return immediately if it's not an appropriate time to poll.
Note that previously, if any joystick/controller was opened, we would poll
every time anyhow, skipping this function.
