Made an arena object to hold arena circles in memory betwen frames

- Added Arena class
- Added returnD3DCOLOR function to Color struct
- Added an Arena pointer to Game class
- Added CreateArena function to direct3D.cpp
- Added prototype to Global.h for CreateArena function
- Fixed comments in Polygon.h
- Changed DrawCircle function to arena->mesh->draw in Render.cpp
- Updated TODO.txt
- Minor changes in all other documents