Tracks: let solve make illegal moves

Not only does it set the outer edges to NOTRACK, but it may also overwrite
any mistakes the user has previously made elsewhere. Otherwise, the entire
solve is rejected ("Solve unavailable" error on Android) if the user has
made a single mistake, which is inconsistent with the other games.

This may be giving a free pass to corrupted moves that occur after a solve,
so this may still want tightening up in some way, but it's still limited to
squares within the grid, so I agree with Ben's assessment that this is
likely not to be exploitable.

Fixes #584

(cherry picked from Android port, commit
33bd14f)

tracks.c

@@ -2429,7 +2429,7 @@ static game_state *execute_move(const game_state *state, const char *move)
             f = (c == 'T' || c == 't') ? S_TRACK : S_NOTRACK;
 
             if (d == 'S') {
-                if (!ui_can_flip_square(ret, x, y, f == S_NOTRACK))
+                if (!ui_can_flip_square(ret, x, y, f == S_NOTRACK) && !ret->used_solve)
                     goto badmove;
                 if (c == 'T' || c == 'N')
                     ret->sflags[y*w+x] |= f;
@@ -2440,7 +2440,7 @@ static game_state *execute_move(const game_state *state, const char *move)
                     unsigned df = 1<<i;
 
                     if (MOVECHAR(df) == d) {
-                        if (!ui_can_flip_edge(ret, x, y, df, f == S_NOTRACK))
+                        if (!ui_can_flip_edge(ret, x, y, df, f == S_NOTRACK) && !ret->used_solve)
                             goto badmove;
                         if (c == 'T' || c == 'N')
                             S_E_SET(ret, x, y, df, f);