[RFC][PATCH xserver] Multi-buffered flips on subsurfaces seg fault in Mesa

[Roman Gilg]

The above patch series only allows flips on a child window with the same
dimensions as its parent xwl_window. For flips on child windows (for example
the video view port of a video player in windowed mode, see here:
http://www.subdiff.de/assets/images/2017-07-28-a-new-beginning.jpg) I wanted
to use Wayland subsurfaces as in the attached patch.

The flipping itself works well on Weston and on KWin. But when shutting down
KWin in the end I get always a segmentation fault in Mesa when cleaning up EGL
via eglTerminate(EGLDisplay) through libepoxy.

I pinpointed the seg fault to be a missing destroyImage function pointer here:
https://cgit.freedesktop.org/mesa/mesa/tree/src/egl/drivers/dri2/egl_dri2.c#n2685

But I don't see currently what component is making the problems here or if the
problem lies in my code.