[warningectomy] Unused variables

diapathrenderer.c: In function 'path_build_arc':
diapathrenderer.c:413:8: warning: unused variable 'ctl' [-Wunused-variable]
   real ctl;
path-math.c: In function 'path_combine':
path-math.c:745:7: warning: unused variable 'total' [-Wunused-variable]
   int total = 0;
       ^
path-math.c:744:13: warning: unused variable 'k' [-Wunused-variable]
   int i, j, k;
             ^
path-math.c:744:10: warning: unused variable 'j' [-Wunused-variable]
   int i, j, k;
          ^
path-math.c:744:7: warning: unused variable 'i' [-Wunused-variable]
   int i, j, k;
       ^
diasvgrenderer.c: In function '_gradient_do':
diasvgrenderer.c:114:10: warning: unused variable 'name' [-Wunused-variable]
   gchar *name = (gchar *)key;
          ^
box.c: In function 'box_draw':
box.c:322:8: warning: unused variable 'radius' [-Wunused-variable]
   real radius;
        ^
shape-export.c: In function 'draw_polygon':
shape-export.c:418:10: warning: unused variable 'style' [-Wunused-variable]
   gchar *style;
          ^
shape-export.c: In function 'add_rectangle_connection_points':
shape-export.c:446:9: warning: unused variable 'center' [-Wunused-variable]
   Point center;
         ^