QOpenGLContext::getProcAddress returns NULL for legacy OpenGL entry points (Windows)

Currently it seems that on Windows platforms QOpenGLContext::getProcAddress is implemented by just calling wglGetProcAddress. Unfortunately wglGetProcAddress returns NULL for all legacy entry points (i.e. even for commonly used and necessary functions like glClear, etc.). I suggest to implement a fallback as it is done for example in SDL_GL_GetProcAddress that resolves an entry point with LoadLibrary("OPENGL32.DLL") and GetProcAddress(...) whenever wglGetProcAddress returns zero. Not only does it make sense to be able to query all OpenGL entry points the same way, but the current behavior also introduces platform specific behavior, as for example glXGetProcAddress doesn't return NULL for these entry points.