GTK 3: handle nontrivial window scale factors.

A user pointed out that if you run a GTK 3 puzzles with "GDK_SCALE=2"
in the environment, the main game drawing area is blurred. That's
because we're choosing the size of our backing Cairo surface based on
the number of _logical_ pixels in the window size, not taking into
account the fact that the non-unit scale factor means the number of
physical pixels is larger. Everything 'works' in the basis - Cairo
happily expands the smaller backing surface into the larger window -
but resolution is lost in the process.

Now we detect the window's scale factor, construct the backing surface
appropriately, and compensate for that scaling when drawing to the
surface and when blitting the surface to the window.

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