refactoring: positioning of mixed_checker addressing duplicateExpression warning

[Sharansrj567]

This pull request addresses a code clarity enhancement in the file graphics.c where the yarr array is utilized to store y-coordinates for drawing polygons. The modification aims to improve the understanding of the code by adjusting the assignment of values to yarr[2] based on the parity of the loop index.

Reasons:

Correction for Checkerboard Pattern: In the context of the surrounding code, this adjustment ensures the correct positioning of the checkerboard pattern by modifying the calculation of the y-coordinate.
Code Clarity: The change is made to address the duplicateExpression warning flagged by cppcheck and to enhance the clarity of the code.
Coding Standards: Additionally, the modified code adheres to the existing coding standards.

Testing:
Manual testing has been performed both before and after making the changes to ensure that the behavior of the code remains unchanged.
A screenshot of the changes made is attached to provide a visual reference for the modification.

[nilason]

With your present proposed changes I get the following result:

By keeping the original logic, with my suggested change, it works (without duplicateExpression warning).