stripped the changes of QOpenGL branch to use QOpenGLWidgets on Qt5.4 or newer.
As a preparation for the move to Core context support, the QOpenGLWidget classes are used on Qt 5.4 or newer.
However i did not yet check compatibility with Qt 5.3 or older
STATUS: 26.5.17 -> GLError on OSX when rendering a cube (not related to these changes). Seems like Qt version 5.7 and 5.8 are the only affected versions this MR is ready to merge imho