There are two programs to be written, the first one simpler (so do it first).
Write a C or C++ program, using the OpenGL library and programmable shaders, that draws several 2-D polygons of different colors, with at least two vertices shared among more than one polygon.
Write a C or C++ program, using the OpenGL library and programmable shaders, that allows the user to create 2-D polygons by clicking with the mouse to indicate vertex coordinates. (You are not required to handle self-intersecting polygons correctly, although I'll be more impressed if you do.)
For a first version, just build a single
polygon from all of the user's mouse clicks.
(I did this in 27 lines of Racket; naturally, it'll take a lot more in
C++ with OpenGL, but on the other hand OpenGL provides a lot of capabilities
that the Racket picturing-programs library doesn't.)
Once you've got that
working, try building multiple polygons: the user should
indicate that the current polygon is finished either by double-clicking,
or by pressing a specified key on the keyboard, or by clicking
“near” the first vertex (your choice which to implement), after
which subsequent mouse-clicks go into a separate polygon.
(This took me 39 lines of Racket.)