d3d11window: Fix for broken dirty rect drawing on Windows 7
DXGI_SWAP_EFFECT_DISCARD cannot be used with dirty rect drawing feature of IDXGISwapChain1::Present(). Note that IDXGISwapChain1 interface is available on Platform Update for Windows 7 and DXGI_SWAP_EFFECT_FLIP_SEQUENTIAL is also the case.
Fixes: #1201 (closed)