Summary:
SharingPlatformContext::swapBuffers needs to make the context current.
For this is calls makeCurrent. Later on this tries to bind a fbo, which
checks calls into the current QOpenGLContext. As we called makeCurrent on
our own platform context Qt doesn't know that the context is current and
returns in the worst case a nullptr and crashes.

This change calls makeCurrent on the QOpenGLContext so that Qt also knows
that this context is current now. The QOpenGLContext calls makeCurrent on
the platform context, so what our code tried to do is still being done.

In addition the method context() in AbstractPlatformContext is renamed to
eglContext to no longer shadow the method in QPlatformContext.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D2582