util_altKeyDown to allow user to choose between scrolling board and
dragging the hint rect when both are possible. add adjustYOffset;
make it and board_setYOffset more tolerant of out-of-bounds inputs and
use that to simplify calling code.
before and after the scroll. Otherwise on platforms where scrolling
is via a bitmove the old position gets scrolled away and never redrawn
leaving ghost tile parts lying around.
