Load a spritesheet texture.
        cell_size is the uniform size of each cell in the spritesheet.

def load_texture(self, file_path, cell_size=(256, 256)):
        """
        Load a spritesheet texture.
        cell_size is the uniform size of each cell in the spritesheet.
        """
        super(SpriteSheet, self).load_texture(file_path)
        self.__cell_bounds = cell_size
        self.__generate_cells()