st/nine: Detach buffers in swapchain dtor.
st/nine: Detach buffers in swapchain dtor.

BackBuffers can survive swapchain dtor if
the user has a reference on them.

The swapchain itself has no reference on the buffer.

Signed-off-by: Axel Davy <2b6ef604b998fb81662f85c329e4fc020eae8e5d@ens.fr>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler