D7734: Add virtual method to Scene to get the EGL/GLX extensions

[Martin Flöser]

graesslin added a dependent revision: D7740: Move SceneOpenGL into a dedicated 
plugin.

REPOSITORY
  R108 KWin

REVISION DETAIL
  https://phabricator.kde.org/D7734

To: graesslin, #kwin, #plasma
Cc: plasma-devel, kwin, bwowk, ZrenBot, progwolff, lesliezhai, ali-mohamed, 
hardening, jensreuterberg, abetts, sebas, apol, mart