Update fnc_mapGridToPos.sqf (#1579)

With check-decrement ordering the while loops execute twice, leaving _minus at -2, then _rvOriginX/_rvOriginY get set to 0 + (abs - 2) - 1 = 1 (and get cached at GVAR(rvOriginX)/GVAR(rvOriginY)), then at line 176 said variables get subtracted from previous calculated coordinates

addons/common/fnc_mapGridToPos.sqf

@@ -120,15 +120,15 @@ if (IS_STRING(_pos)) then {
             private _check = _start;
             private _minus = 0;
             while {_check == _start} do {
-                _check = format["%1", mapGridPosition [0, _minus]];
                 _minus = _minus - 1;
+                _check = format["%1", mapGridPosition [0, _minus]];
             };
             _rvOriginY = _rvOriginY+(abs _minus)-1;
             _minus = 0;
             _check = _start;
             while {_check == _start} do {
-                _check = format["%1", mapGridPosition [_minus, 0]];
                 _minus = _minus - 1;
+                _check = format["%1", mapGridPosition [_minus, 0]];
             };
             _rvOriginX = _rvOriginX+(abs _minus)-1;
         };