Add support for getProcAddress in QOpenGLContext

Hi,

Currently, the PySide2 implementation of QOpenGLContext lacks the function getProcAddress ( see: https://doc.qt.io/qt-5/qopenglcontext.html#getProcAddress ).

This function is critical for everyone who wants to plug their own engine to the Qt's OpenGL window&context instead of using the QOpenGLFunctions.

As an example, my current 3D engine is programmed in Rust, and i call it via python (using ffi). The engine only has to handle sending the OpenGL commands. All the window and context creation is handle by external library ( in this case here, Qt, but this architecture allow to change the window/context management without impacting the rendering aspect ). But, to be able to use the OpenGL functions, the engine need to be able to load the function pointer from the context.

To do this in C++ Qt, we can pass the getProcAddress function to the engine. But this function is not accessible in PySide2 (nor PyQT5 also) making it impossible to use the Qt OpenGL context outside of Qt's specific OpenGL functions.