d3d11 backend for the Render API

It would be very nice to have d3d11 interface accessible within libmpv/render.h for external applications.

It could be a solution which allows to get directx 2D texture/shader resource view handles which can be used in a user application with its own dx11 device/context (i.e. shared immutable or dynamic texture). The final video frames are to be rendered into that texture (app tells needed resolution to mpv)

The alternative solution is to pass a d3d11 device from host app to mpv and then either
(a) again, create a texture inside mpv and pass back to an app or
(b) do it similar to how GL render API works now, i.e. render to a user-provided render target (which I don't like that much though, since having a texture is more convenient to me, though it still serves its needs)

I started do implement this feature on my own some time ago, but I'm a bit lost inside mpv's internal rendering structures, and debugging is quite tricky under windows (although I have a pdb file which I can use for debugging under Visual Studio, but it's not ideal).

Just wondering if someone can guide me a bit?

Start at libmpv/render.h and trace the existing Render API into mpv’s internal rendering structures. Use the Windows Visual Studio debugging setup described in the issue as needed; done means defining a d3d11 interface that supports the requested external texture or device flow and renders final frames at the application’s requested resolution.