app: don't use PROJ_ROUND() for pixel-adjusting canvas item drawing

When PROJ_ROUND()ing e.g. 3.8, it ends up at 4, then we added the 0.5
offset to draw a nice cairo line in the middle of the pixel,
effectively drawing a line that's meant to be at 3.8 at 4.5.  Instead,
we now use floor(x)+0.5 now which snaps the above example to 3.5.

Also, calculate arcs like we calculate rectangles (transform the arc's
bounding box and pixel-align that, then recalculate the center), so
arcs properly align with rectangles.

app/display/gimpcanvasarc.c

@@ -235,10 +235,6 @@ gimp_canvas_arc_transform (GimpCanvasItem   *item,
   gdouble               x1, y1;
   gdouble               x2, y2;
 
-  gimp_display_shell_transform_xy_f (shell,
-                                     private->center_x,
-                                     private->center_y,
-                                     center_x, center_y);
   gimp_display_shell_transform_xy_f (shell,
                                      private->center_x - private->radius_x,
                                      private->center_y - private->radius_y,
@@ -248,6 +244,14 @@ gimp_canvas_arc_transform (GimpCanvasItem   *item,
                                      private->center_y + private->radius_y,
                                      &x2, &y2);
 
+  x1 = floor (x1);
+  y1 = floor (y1);
+  x2 = ceil (x2);
+  y2 = ceil (y2);
+
+  *center_x = (x1 + x2) / 2.0;
+  *center_y = (y1 + y2) / 2.0;
+
   *radius_x = (x2 - x1) / 2.0;
   *radius_y = (y2 - y1) / 2.0;
 

app/display/gimpcanvascorner.c

@@ -267,10 +267,10 @@ gimp_canvas_corner_transform (GimpCanvasItem   *item,
   rw -= rx;
   rh -= ry;
 
-  rx = PROJ_ROUND (rx) + 0.5;
-  ry = PROJ_ROUND (ry) + 0.5;
-  rw = PROJ_ROUND (rw) - 1.0;
-  rh = PROJ_ROUND (rh) - 1.0;
+  rx = floor (rx) + 0.5;
+  ry = floor (ry) + 0.5;
+  rw = ceil (rw) - 1.0;
+  rh = ceil (rh) - 1.0;
 
   top_and_bottom_handle_x_offset = (rw - private->corner_width)  / 2;
   left_and_right_handle_y_offset = (rh - private->corner_height) / 2;

app/display/gimpcanvashandle.c

@@ -406,8 +406,8 @@ gimp_canvas_handle_transform (GimpCanvasItem   *item,
       break;
     }
 
-  *x = PROJ_ROUND (*x) + 0.5;
-  *y = PROJ_ROUND (*y) + 0.5;
+  *x = floor (*x) + 0.5;
+  *y = floor (*y) + 0.5;
 }
 
 static void

app/display/gimpcanvasline.c

@@ -200,10 +200,10 @@ gimp_canvas_line_transform (GimpCanvasItem   *item,
                                      private->x2, private->y2,
                                      x2, y2);
 
-  *x1 = PROJ_ROUND (*x1) + 0.5;
-  *y1 = PROJ_ROUND (*y1) + 0.5;
-  *x2 = PROJ_ROUND (*x2) + 0.5;
-  *y2 = PROJ_ROUND (*y2) + 0.5;
+  *x1 = floor (*x1) + 0.5;
+  *y1 = floor (*y1) + 0.5;
+  *x2 = floor (*x2) + 0.5;
+  *y2 = floor (*y2) + 0.5;
 }
 
 static void

app/display/gimpcanvaspolygon.c

@@ -213,8 +213,8 @@ gimp_canvas_polygon_transform (GimpCanvasItem   *item,
                                          &points[i].x,
                                          &points[i].y);
 
-      points[i].x = PROJ_ROUND (points[i].x) + 0.5;
-      points[i].y = PROJ_ROUND (points[i].y) + 0.5;
+      points[i].x = floor (points[i].x) + 0.5;
+      points[i].y = floor (points[i].y) + 0.5;
     }
 }
 

app/display/gimpcanvasrectangle.c

@@ -232,10 +232,10 @@ gimp_canvas_rectangle_transform (GimpCanvasItem   *item,
     }
   else
     {
-      *x = PROJ_ROUND (*x) + 0.5;
-      *y = PROJ_ROUND (*y) + 0.5;
-      *w = PROJ_ROUND (*w) - 1.0;
-      *h = PROJ_ROUND (*h) - 1.0;
+      *x = floor (*x) + 0.5;
+      *y = floor (*y) + 0.5;
+      *w = ceil (*w) - 1.0;
+      *h = ceil (*h) - 1.0;
     }
 }