SwapChainRenderTarget  leak resources.

This one leak windowHandle resources. ('User objects' at Task Manager).
After 10000 or so the App crash.

`SharpDX.Utilities.Dispose()` can't be used on `_swapChain`, maybe
remove `readonly` modifier?
Remove readonly & call SharpDX.Utilities.Dispose() on _swapChain
Merge pull request #2901 from nkast/SwapChainRenderTarget_UserOBjectsLeak

SwapChainRenderTarget  leak resources