Pass function ptr to resolve glFunctions to initGL

Summary:
KWin still resolves some OpenGL function pointers. For that it needs to
use either eglGetProcAddress or glxGetProcAddress. With other words the
method to resolve needs to know whether it is egl or glx and needs both
a dependency to egl and glx. Especially the dependency to glx is ugly as
that pulls in XLib into our library.

The way so far was to pass an enum value to the initGL method to know
whether it's EGL or GLX. With this change the enum value is removed and
replaced by a function pointer to resolve the methods.

This simplifies the resolve code and allows to completely remove the glx
variant we still had in the library. Thus kwinglutils library is now glx
and XLib free.

Test Plan: nested KWin with OpenGL/EGL still works

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3336