Clean up resolving of OpenGL functions on Windows

Always try both e/wglGetProcAddress and ::GetProcAddress to
resolve the methods. Like this QOpengGLContext::getProcAddress is
able to return any OpenGL entry point, and we can both simplify
the code we have in the QPA backend as well as get rid of windows
specific code paths in Qt Gui.

Task-number: QTBUG-39531
Change-Id: I1ddf1b0974f69b56b219a619655b723eb0134b14
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>