surface: allow placing subsurfaces below parent

Prior to this commit, subsurfaces could only be placed above their
parent. Any place_{above,below} request involving the parent would
fail with a protocol error.

However the Wayland protocol allows using the parent surface in the
place_{above,below} requests, and allows subsurfaces to be placed
below their parent.

Weston's implementation adds a dummy wl_list node in the subsurface
list. However this is potentially dangerous: iterating the list
requires making sure the dummy wl_list node is checked for, otherwise
memory corruption will happen.

Instead, split the list in two: one for subsurfaces above the parent,
the other for subsurfaces below.

Tested with wleird's subsurfaces demo client.

Closes: https://github.com/swaywm/wlroots/issues/1865

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