Some cleanup & comments
Switch from QGLContext to QOpenGLContext

QOpenGLContext is newer and I believe QGL* stuff is being deprecated in
favor of QOpenGL*
Store QOpenGLFunctions/Context on instances

I also did the same thing to the Renderer class, by storing an instance
on the Scene.  This doesn't alleviate the crashing though, and I may
undo this.

I also simplified the extensions code for ARB_multitexture, and
anistropy.  I no longer use QOpenGLFunctions_1_4 to get the extension
functions.  These changes stop the crashing when trying to load multiple
meshes.

Still has paint errors however.
Fixed file dragging / double clicking

Path to file is now interpreted correctly by the application.