Pull term references out of xdrawcursor

Gradually reducing x.c dependency on Term object.  Old and new cursor
glyph/position are passed to xdrawcursor.  (There may be an opportunity
to refactor further if we can unify "clear old cursor" and "draw new
cursor" functionality.)

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>

st.h

@@ -82,8 +82,10 @@ typedef struct {
 	int col;      /* nb col */
 	Line *line;   /* screen */
 	Line *alt;    /* alternate screen */
-	int *dirty;  /* dirtyness of lines */
+	int *dirty;   /* dirtyness of lines */
 	TCursor c;    /* cursor */
+	int ocx;      /* old cursor col */
+	int ocy;      /* old cursor row */
 	int top;      /* top    scroll limit */
 	int bot;      /* bottom scroll limit */
 	int mode;     /* terminal mode flags */