Drop Shadow for Mobjects

## Description of proposed feature
Adding drop shadow to mobjects.
The addition of a drop shadow feature for mobjects in Manim would greatly enhance the visual appeal and depth of animations created using the library.

The issue proposes drop shadows for Mobjects but does not identify any files, tests, rendering entry points, or expected behavior. Start by locating Mobject rendering code and existing visual-effect tests, then clarify the shadow appearance, supported Mobject types, and how completion will be verified.

Written by the indexing model from the issue text.