Qt 5 OpenGL updates

[a-y-khan]

• Update to new non-native OpenGL widget class(QOpenGLWidget), legacy QGLWidget class still supported
• Update context sharing between threads on Mac OS X, Windows (rendering not threaded on Linux)
• Port QtTransferFunctionWidget, QtRenderWidget (based on legacy QGLWidget class)
• GLEW/Qt workarounds, may also be worth trying GLEW 1.13 to see if works any better with Qt

[a-y-khan]

See also #119.

[dcwhite]

Interesting link:
https://forum.qt.io/topic/92949/migration-from-qglwidget-to-qopenglwidget