QGLPixelBuffer and others require conditional compilation on libQtOpenGL being present

Recent refactoring in the qt5/qt3d repo has removed the dependencies on libQtOpenGL and libQtWidgets, which is good because it means that all platforms are supported, not just desktops.

However the conditional compilation code was bad as it changed the library in a non-binary compatible way depending on if those supports were there.

The casualties from removing this is QGLPixelBuffer which was about the only thing using it, and the copyImage function on QGLTexture2D. Neither of these are core and are not really used in the current examples, so just remove them and get rid of conditional compilation.