Centralise initial clearing of the puzzle window.

I don't know how I've never thought of this before! Pretty much every
game in this collection has to have a mechanism for noticing when
game_redraw is called for the first time on a new drawstate, and if
so, start by covering the whole window with a filled rectangle of the
background colour. This is a pain for implementers, and also awkward
because the drawstate often has to _work out_ its own pixel size (or
else remember it from when its size method was called).

The backends all do that so that the frontends don't have to guarantee
anything about the initial window contents. But that's a silly
tradeoff to begin with (there are way more backends than frontends, so
this _adds_ work rather than saving it), and also, in this code base
there's a standard way to handle things you don't want to have to do
in every backend _or_ every frontend: do them just once in the midend!

So now that rectangle-drawing operation happens in midend_redraw, and
I've been able to remove it from almost every puzzle. (A couple of
puzzles have other approaches: Slant didn't have a rectangle-draw
because it handles even the game borders using its per-tile redraw
function, and Untangle clears the whole window on every redraw
_anyway_ because it would just be too confusing not to.)

In some cases I've also been able to remove the 'started' flag from
the drawstate. But in many cases that has to stay because it also
triggers drawing of static display furniture other than the
background.

bridges.c

@@ -2987,9 +2987,6 @@ static void game_redraw(drawing *dr, game_drawstate *ds,
 
     /* Clear screen, if required. */
     if (!ds->started) {
-        draw_rect(dr, 0, 0,
-                  TILE_SIZE * ds->w + 2 * BORDER,
-                  TILE_SIZE * ds->h + 2 * BORDER, COL_BACKGROUND);
 #ifdef DRAW_GRID
         draw_rect_outline(dr,
                           COORD(0)-1, COORD(0)-1,