Define explicit macros instead of fooling the user with a fake main.
Eliminate some warnings in `OpenGLWindow.cpp`.
Fix graphics examples by using explicit macro.
Remove extraneous variable.
Eliminate CMake warning.
Break lines up to 80 characters.
Merge pull request #98 from DO-CV/maint-eliminate-warnings-in-graphics-module

[MAINT] Eliminate compiler warnings in Graphics module.