Fix some build warnings

private field 'm_dwCookie' is not used
'return' will never be executed
result of comparison of unsigned enum expression < 0 is always false
'FlushDC' overrides a member function but is not marked 'override'
potentially uninitialized local variable 'bound' used

samples/dll/sdk_exe.cpp

@@ -82,8 +82,6 @@ LRESULT CALLBACK MainWndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
         default:
             return DefWindowProc(hwnd, msg, wParam, lParam);
     }
-
-    return 0;
 }
 
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