In the builds I make, the "directx" drivers in SDL are disabled, so it's just windib. From that, DOSBox-X offers output=surface, output=opengl, and output=direct3d.
The async code changes were made in order to accomplish minimal changes to the SDL library. The only change is that the SDL window managed by windib is a child window of a parent window managed by another thread, so that resize, move and menu events to the parent window do not block DOSBox-X or SDL. Debug output is still sent to the console window, which is maintained by Windows.
The parent and child windows are set up so that a WM_CLOSE sent to the parent is handled by the child. If the parent is given a WM_DESTROY, then the child window is destroyed as well.
DOSBox-X project: more emulation better accuracy.
DOSLIB and DOSLIB2: Learn how to tinker and hack hardware and software from DOS.