Split back-endish part off drawing code and move to board.c

Split back-endish part off drawing code and move to board.c

Split DrawSquare in front-end and back-end part
Back-endize DrawHighlights and DrawPosition
 The already existing wrapper FlashDelay is used as a kludge to do an XSync.
 We now pass a code for the line color GC to the highight routine. New
 wrappers are DrawGrid() and DrawBorder().
Back-endize atomic explosions
Back-endize DrawArrow and DrawDragPiece
 DrawDragPiece now calls DrawOneSquare with argument EmptySquare in stead
 of the X-type referring BlankSquare. A new wrapper DrawPolygon fixes
 DrawArrowBetweenPoints.
Move board-drawing logic to new file board.c
 All X-independent drawing logic (board, squares, arrows, highlights,
 some animation stuff) has been extracted from xboard.c, and moved to
 a new file board.c, which is back-end (but unshared with WinBoard).
 A new header board.h defines the cross-references (which of course
 required some functions to be no longer declared as static).
  A bit of code was moved from DragPieceBegin to BeginAnimation, which
 now has an extra argument to indicate which piece should appear from
 under a dragged piece. This makes DragPieceBegin free of XCopyArea calls,
 so it could be moved too.
Make board.c truly back-end
 Get rid of all X data types. For this the AnimState struct had to be
 redefined: the GC and Pixmap was taken out and put in front-end arrays
 indexed by anim agent (game and player). For this indexing a new enum
 was defined. The XPoint type was also replaced by our own type of an
 int pair (which is nasty, because the int size could be different).
Move more animation code to board.c