making all four corners of the grid sharp instead of round (#6993)

# Simple visual change 

Currently, the grid has 3 round corners and 1 sharp corner. It seems
like bambu lab hard coded this specifically to fit their bed model. As a
non-bambu user, I dont like how the corners look.
![Screenshot 2024-10-02
152326](https://github.com/user-attachments/assets/ba905ed1-3557-488e-adb2-ea1ea93bb1cc)
![Screenshot 2024-10-02
152435](https://github.com/user-attachments/assets/7a2ec002-87c3-46ab-b5ff-8174c20890e3)

Another point I'd like to bring up is that rounded corners do not match
the boundary detection box.


## Tests
Tested locally on windows and macos.

src/slic3r/GUI/PartPlate.cpp

@@ -2541,59 +2541,9 @@ void PartPlate::generate_print_polygon(ExPolygon &print_polygon)
  }
  };
 
- int points_count = 8;
- if (m_shape.size() == 4)
- {
- //rectangle case
- for (int i = 0; i < 4; i++)
- {
- const Vec2d& p = m_shape[i];
- Vec2d center;
- double start_angle, stop_angle, radius_x, radius_y, radius;
- switch (i) {
- case 0:
- radius = 5.f;
- center(0) = p(0) + radius;
- center(1) = p(1) + radius;
- start_angle = PI;
- stop_angle = 1.5 * PI;
- compute_points(center, radius, start_angle, stop_angle, points_count);
- break;
- case 1:
- print_polygon.contour.append({ scale_(p(0)), scale_(p(1)) });
- break;
- case 2:
- radius_x = (int)(p(0)) % 10;
- radius_y = (int)(p(1)) % 10;
- radius = (radius_x > radius_y)?radius_y: radius_x;
- if (radius < 5.0)
- radius = 5.f;
- center(0) = p(0) - radius;
- center(1) = p(1) - radius;
- start_angle = 0;
- stop_angle = 0.5 * PI;
- compute_points(center, radius, start_angle, stop_angle, points_count);
- break;
- case 3:
- radius_x = (int)(p(0)) % 10;
- radius_y = (int)(p(1)) % 10;
- radius = (radius_x > radius_y)?radius_y: radius_x;
- if (radius < 5.0)
- radius = 5.f;
- center(0) = p(0) + radius;
- center(1) = p(1) - radius;
- start_angle = 0.5 * PI;
- stop_angle = PI;
- compute_points(center, radius, start_angle, stop_angle, points_count);
- break;
- }
- }
- }
- else {
- for (const Vec2d& p : m_shape) {
- print_polygon.contour.append({ scale_(p(0)), scale_(p(1)) });
+ for (const Vec2d& p : m_shape) {
+ print_polygon.contour.append({scale_(p(0)), scale_(p(1))});
  }
- }
 }
 
 void PartPlate::generate_exclude_polygon(ExPolygon &exclude_polygon)