kwinglutils: Clean up persistent vbo fences at the start of the frame

With a persistent vbo, kwin will allocate one big enough buffer and
allocate memory out of it.

In order to prevent overwriting vertex buffer data that is currently
being accessed by the GPU, fences are inserted at the end of frame.

The signaled fences are destroyed after the buffer swap operation, which
seems a bit odd because the just inserted fence most likely won't be
signaled. Perhaps it's a historical artifact?

This change rearranges fence cleanup so it's performed right before
starting a new frame. With it, kwin will most likely re-use the
previously used memory chunk because there will be plenty of time for
the fence to become signaled.

Another motivation behind this change is to make refactoring SceneOpenGL
code easier. As is, m_backend->endFrame() is wrapped in
GLVertexBuffer::endOfFrame() and GLVertexBuffer::framePosted(). With
that, the Compositor can't call m_backend->endFrame(), which can be
desired for cleaning up render backend abstractions.