Axes.get_riemann_rectangles() has wrong widths

The rectangle widths when using Axes.get_riemann_rectangles() do not 'scale' to the size of the axis. The height is correctly scaled.

The issue is here: https://github.com/3b1b/manim/blob/c45ff910f0790a3aeeccf10c855910f231a7f9b5/manimlib/mobject/coordinate_systems.py#L250

Here is my fix:
```python
left, right = self.c2p(x0, 0)[0], self.c2p(x1, 0)[0]
rect = Rectangle(width=right - left, height=height)
```