Sprite to polygon conversion improvements

- No reduced Rect in march square algorithm, it was causing inconsistent cases near the borders and made the outline less accurate
- Ignore invalid generated polygons (under 3 points) to avoid unnecessary errors and crashes
- Error popup only when no polygon could be generated at all
- Added option to shrink pixels (to get rid of small separate islands)
- Fixed polygon preview (lines were sometimes not showing along the borders)

Fixes #32564, #29267

editor/plugins/sprite_editor_plugin.h

@@ -67,7 +67,8 @@ class SpriteEditor : public Control {
 	Vector<int> computed_indices;
 
 	SpinBox *simplification;
-	SpinBox *island_merging;
+	SpinBox *grow_pixels;
+	SpinBox *shrink_pixels;
 	Button *update_preview;
 
 	void _menu_option(int p_option);