direct3d11: clear the D3D11 view before writing on it

In the windows store app the D3D11 view is reused between files
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/video_output/msw/direct3d11.c

@@ -809,6 +809,9 @@ static void Display(vout_display_t *vd, picture_t *picture, subpicture_t *subpic
 {
     vout_display_sys_t *sys = vd->sys;
 
+    FLOAT blackRGBA[4] = {0.0f, 0.0f, 0.0f, 1.0f};
+    ID3D11DeviceContext_ClearRenderTargetView(sys->d3dcontext, sys->d3drenderTargetView, blackRGBA);
+
     /* no ID3D11Device operations should come here */
 
     ID3D11DeviceContext_OMSetRenderTargets(sys->d3dcontext, 1, &sys->d3drenderTargetView, sys->d3ddepthStencilView);