getProcAddress() should return a function pointer

The return type of QGLContext::getProcAddress() should be a function pointer and not a data pointer. The wrapped function glXGetProcAddress() has the correct prototype, but for some reason the QGLContext method deviates from the correct GLX API. According to the C and C++ standards, it is valid to cast between different function pointer types, as long as the actual call is made through a pointer with the original signature of the function pointed to. However, the standard explicitly does not make the same guarantee for the conversion from a function pointer to a non-function pointer and back.

Strictly speaking, it should be a pointer to a function with C linkage:

extern "C"

...
QGLProc getProcAddress(const QString&);

This is an API bug, and the invalid type conversion will cause compilers to complain with the right warning settings.