Readded TestEngine.h and shapestest.cpp
Removed PlainShader, added shapestest source to test/CMakeLists
Updated shapestest cube init function
Removed void* parameter from GenericCallback, added UpdateTest callback in shapestest source
Added Pre/On/PostRenderTest callbacks and ShutDown callback to shapestest source
Added missing return statements in shapestest source
Fixed the double PreRenderCallback being set in shapestest
Removed cube.cpp
Added TestEngine include to rectangle.cpp & removed cube.cpp from test/CMakeLists