Keep swapchain while its backbuffer is in use.

[Michael Karcher]

DirectX 9 SDK Documentation says that the surfaces acquired by
IDirect3DSwapChain9::GetBackBuffer must be released before the swap
chain may be released. The back buffer of the new chain is not released
while it is used as render target, so we have to keep the swapchain alive.

This fixes the warn+heap errors/crashes of d3d9:stateblock, and should
close the bug about differences with warn+heap enabled.
---
 dlls/d3d9/tests/stateblock.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)
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