Create a child window as part of GLX surfaces.

Create a child window as part of GLX surfaces.

Some NVIDIA drivers don't allow deleting GLX windows separately from their
parent X windows. Work around this by creating a child X window to the
window passed in, creating the GLX window against the child window, and
then destroying the child window to destroy the GLX window.

Because the child window covers the parent window, it will receive all
expose events instead of the parent. Set up an explicit forwarding of
these events from the child window to the parent window.

Note that this patch does not ever destroy the child window -- that will
come in a separate patch.

BUG=145600