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Forbid impossible moves in Bridges

Specifically, a bridge or a non-bridge must connect two islands that
differ in precisely one co-ordinate.  Without this, a save file that
tries to connect or disconnect two non-orthogonal islands will cause
"island_join: Assertion `!"island_join: islands not orthogonal."'
failed."

Here's a save file demonstrating the problem:

SAVEFILE:41:Simon Tatham's Portable Puzzle Collection
VERSION :1:1
GAME    :7:Bridges
PARAMS  :13:3x3i30e10m2d0
CPARAMS :13:3x3i30e10m2d0
DESC    :6:b1c1a2
NSTATES :1:2
STATEPOS:1:2
MOVE    :10:L0,2,2,0,1

--- a/bridges.c

+++ b/bridges.c

@@ -2571,6 +2571,8 @@

                 goto badmove;

             if (!INGRID(ret, x1, y1) || !INGRID(ret, x2, y2))

                 goto badmove;

+            /* Precisely one co-ordinate must differ between islands. */

+            if ((x1 != x2) + (y1 != y2) != 1) goto badmove;

             is1 = INDEX(ret, gridi, x1, y1);

             is2 = INDEX(ret, gridi, x2, y2);

             if (!is1 || !is2) goto badmove;

@@ -2582,6 +2584,7 @@

                 goto badmove;

             if (!INGRID(ret, x1, y1) || !INGRID(ret, x2, y2))

                 goto badmove;

+            if ((x1 != x2) + (y1 != y2) != 1) goto badmove;

             is1 = INDEX(ret, gridi, x1, y1);

             is2 = INDEX(ret, gridi, x2, y2);

             if (!is1 || !is2) goto badmove;