On Wayland, internal windows that use OpenGL are rendered into fbos,
which are later handed over to kwin. In order to achieve that, our QPA
creates OpenGL contexts that share resources with the scene's context.

The problems start when compositing has been restarted. If user changes
any compositing settings, the underlying render backend will be
reinitialized and with it, the scene's context will be destroyed. Thus,
we no longer can accept framebuffer objects from internal windows.

This change addresses the framebuffer object sharing problem by adding
a so called global share context. It persists throughout the lifetime of
kwin. It can never be made current. The scene context and all contexts
created in our QPA share resources with it.

Therefore we can destroy the scene OpenGL context without affecting
OpenGL contexts owned by internal windows, e.g. the outline visual or
tabbox.

It's worth noting that Qt provides a way to create a global share
context. But for ...