st/mesa: add a winsys buffers list in st_context

st/mesa: add a winsys buffers list in st_context

Commit a5e733c6b52e93de3000647d075f5ca2f55fcb71 fixes the dangling
framebuffer object by unreferencing the window system draw/read buffers
when context is released. However this can prematurely destroy the
resources associated with these window system buffers. The problem is
reproducible with Turbine Demo running with VMware driver. In this case,
the depth buffer content was lost when the context is rebound to a
drawable.

To prevent premature destroy of the resources associated with
window system buffers, this patch maintains a list of these buffers in
the context, making sure the reference counts of these buffers will not
reach zero until the associated framebuffer interface objects no
longer exist. This also helps to avoid unnecessary destruction and
re-construction of the resources associated with the framebuffer.

Fixes VMware bug 1909807.

Reviewed-by: Brian Paul <brianp@vmware.com>