Unlike other axis, they don't default to 0, but instead start at -32768 and go to +32767 when pressed. This confuses the
current ES code axis code. As a workaround, we get the initial value and if it is -32767, we add 32767 and divide by two. This gives it
a range that can be handled with the current code (from 0 to 32767). Note on my X11 machine, I had to plug the joystick in after ES
was loaded or it get 0 as the initial axis value for the triggers. This seems ok on the RPI though, so possible some SDL issue.
With this change on a 360 controller the triggers should be correctly detected as +2 and +5 without seeing two presses for each trigger press.