SDL mutex and timer fixes

Team,

It's an ugly hack, but I wrapped put_pixel and and get_pixel of SDL with
a mutex as well as the overlay_sdl::display() method, this is hardly
ideal, but there are no more Xlib errors.  Perhaps we can discuss how to
more intelligently place these mutex lock/unlock calls in the SDL
surface, but it works now.  And since the SDL surface is only there to
get something running quickly, I am not too worried about it.

Second, at times the system will report a time_value that is earlier
than the previous one.  This caused problems int he time_value
operator-, I fixed those and timers work again.

Darren