Added grid_lines attribute to Rectangle to add individual styling to the grid lines (#3428)

* Added 'grid_line_stroke_width' parameter in Rectangle

* Added 'grid_lines' (VGroup) attribute to 'Rectangle' class

---------

Co-authored-by: Tristan Schulz <[email protected]>

Author: RobinPH

manim/mobject/geometry/polygram.py

@@ -598,8 +598,10 @@ class RectangleExample(Scene):
             def construct(self):
                 rect1 = Rectangle(width=4.0, height=2.0, grid_xstep=1.0, grid_ystep=0.5)
                 rect2 = Rectangle(width=1.0, height=4.0)
+                rect3 = Rectangle(width=2.0, height=2.0, grid_xstep=1.0, grid_ystep=1.0)
+                rect3.grid_lines.set_stroke(width=1)
 
-                rects = Group(rect1,rect2).arrange(buff=1)
+                rects = Group(rect1, rect2, rect3).arrange(buff=1)
                 self.add(rects)
     """
 
@@ -618,6 +620,8 @@ def __init__(
         self.stretch_to_fit_width(width)
         self.stretch_to_fit_height(height)
         v = self.get_vertices()
+        self.grid_lines = VGroup()
+
         if grid_xstep is not None:
             from manim.mobject.geometry.line import Line
 
@@ -633,7 +637,7 @@ def __init__(
                     for i in range(1, count)
                 )
             )
-            self.add(grid)
+            self.grid_lines.add(grid)
         if grid_ystep is not None:
             grid_ystep = abs(grid_ystep)
             count = int(height / grid_ystep)
@@ -647,7 +651,10 @@ def __init__(
                     for i in range(1, count)
                 )
             )
-            self.add(grid)
+            self.grid_lines.add(grid)
+
+        if self.grid_lines:
+            self.add(self.grid_lines)
 
 
 class Square(Rectangle):