shell: Fix calculation of center point in surface rotation

Make sure the center point of a rotation is not rounded to an
integer. It makes the calculation consistent with others in the
shell. It also ensures surfaces rotated 180 degrees are at the
exact same place.

src/shell.c

@@ -2885,8 +2885,8 @@ surface_rotate(struct shell_surface *surface, struct weston_seat *seat)
 		return;
 
 	weston_surface_to_global_float(surface->surface,
-				       surface->surface->geometry.width / 2,
+				       surface->surface->geometry.width * 0.5f,
-				       surface->surface->geometry.height / 2,
+				       surface->surface->geometry.height * 0.5f,
 				       &rotate->center.x, &rotate->center.y);
 
 	dx = wl_fixed_to_double(seat->pointer->x) - rotate->center.x;