made a crude version of the 'refinery' object
Redid digging a bit.

It's still hacky, but less so.
If you're positioned to dig, it shows an indicator.
pressing "c" digs that tile, pressing "v" drops in
   empty space.
Changed digging again

Current version: On a single keypress, turn rock into
   rubble or send rubble flying backwards.
Made the main robot into a floater again.

With the reduced gravity and still no friction,
   the pyramid shape was annoying because it was
   so easy to zoom into the air and not come down
   for a while.
Made hacky conveyor belts:

They always point in the +x direction.
They can move a single rubble tile up an incline,
   but they can get clogged by more than one that are
   too close to each other.
hack: made things more visible

Since we're not using the lighting code right now,
   it makes more sense to have everything lit
   than everything dark
wrote that you can make conveyors
fix build with GCC 4.6
use 'override' keyword to check/document which methods are overrides
Re-enable OpenGL thread by default

Nouveau seems to be fixed for me these days and not
have the performance problem with rendering from a separate
thread anymore.  And that was the only reason that it
wasn't enabled by default.
Improve quitting code & comments.

Now we quit via Qt's event loop.

More patch next.
Make quitting more robust.

Now we indeed close the OpenGL thread when quitting
any time that it exists and we quit.

This (hopefully) makes buggy OpenGL implementations happier.
fix comment
clean up OpenGL state
don't check for OpenGL when there's no GUI anyway
add mouse input infrastructure

clicking on the window makes it capture the cursor.
notes
Allow you to control 'robot's facing using the mouse.

TODO we should allow controlling most anything's facing this way.

This commit also fixes the main.cpp mouse infrastructure a bit.
add mouse button support as an input key