Shuffle OpenGL code to ensure a context exists

Summary:
The existing code made several glDoSomething() calls before
KisOpenGLCanvas had entered initializeGL() for the first time.

This moves calls made to create sync objects and compile shaders into
initializeGL. KisOpenGL has two-part initialization step; the first part
is called in main(), the second part is called inside KisOpenGLCanvas2.

Reviewers: dkazakov, rempt

Maniphest Tasks: T360

Differential Revision: https://phabricator.kde.org/D234