D3D11: Fixed the DXGI_SWAP_EFFECT_DISCARD fallback swap chain. (#3997)

The retry kept DXGI_SCALING_NONE, and createSwapChain/resizeBuffers
added DXGI_SWAP_CHAIN_FLAG_ALLOW_TEARING for every swap effect; both
are flip-model only, so the fallback failed with DXGI_ERROR_INVALID_CALL
and a bitblt chain could not be resized.

src/renderer_d3d11.cpp

@@ -1208,6 +1208,8 @@ namespace bgfx { namespace d3d11
 
 							m_scd.bufferCount = m_swapBufferCount;
 							m_scd.swapEffect  = m_swapEffect;
+							// DXGI_SCALING_NONE is only valid for flip-model swap chains.
+							m_scd.scaling     = DXGI_SCALING_STRETCH;
 							hr = m_dxgi.createSwapChain(m_device
 								, m_scd
 								, &mainFrameBuffer().m_swapChain