Break LoadScene out into loader.{cpp,h}, rename ::sptr to Ptr

Move LoadScene out to loader.cpp and loader.h
Don't include builtin_loader.h and trisrc_loader.h in spin.cpp

Change all ...::sptr to just ...Ptr for keystrokes

Move CheckOpenGL() into drawable.cpp

Add some ideas for directions for development to notes

Set gSceneStartTime in main(), rather than LoadScene