Fix pilot controls.
Generate PNG versions of gamepad images.
Improve gamepad docs.

Use PNG images; add some text.
Clean up button highlighting.

Fix names of buttons (they're bumpers, not triggers).  Generalize the
highlighting so it could be used for other buttons.
Allow gamepad to disable autopilot.
Show and use gamepad buttons in interface.

Includes a data update to include the names for the keys.  At present,
all interfaces are assumed to use keyboard labels when the interface
loads, and the screen switches to gamepad labels if a gamepad input is
detected.  This is often not correct.
Show gamepad controls when appropriate.

Make it the responsibility of the VideoDriver to track whether or not a
gamepad is being used.  This persists between different cards, so we can
make the right decision immediately.
Merge branch 'gamepad' into develop