Added rbg_gradient.cpp for SDL gradient graphics

[dawidg81]

Added simple c++ example using SDL that makes a pixel buffer array. RGB Gradient is drawn on the pixel buffer. The pixel buffer is displayed using SDL.

Description of Change

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes:
may be reviewed and improved. Not the best quality