Add validate_params bounds checks in a few more games.

Ben tells me that his recent work in this area was entirely driven by
fuzzing: he added bounds checks in validate_params when the fuzzer had
managed to prove that the lack of them allowed something buggy to
happen.

It seemed worth doing an eyeball-review pass to complement that
strategy, so in this commit I've gone through and added a few more
checks that restrict the area of the grid to be less than INT_MAX.

Notable in this commit: cube.c had to do something complicated because
in the triangular-grid modes the area isn't calculated as easily as
w*h, and Range's existing check that w+h-1 < SCHAR_MAX is sufficient
to rule out w*h being overlarge _but_ should be done before w*h is
ever computed.

filling.c

@@ -188,6 +188,8 @@ static const char *validate_params(const game_params *params, bool full)
 {
     if (params->w < 1) return "Width must be at least one";
     if (params->h < 1) return "Height must be at least one";
+    if (params->w > INT_MAX / params->h)
+        return "Width times height must not be unreasonably large";
 
     return NULL;
 }