Draft: [RFC] add libvlc imem access demux API + D3D11 example

Right now the imem-access can be used to provide raw data, to read from memory or an external source. There is also an imem variant that allows feeding blocks of data but it's not exposed or usable via libvlc.

This new API allows a simple way to feed blocks of data to an access demuxer. There's an example provided with D3D11 Resource blocks of data. It is not as exhaustive as what imem.c provided, but it should probably cover most use cases for that (providing video frames).

There's a new sample file demonstrating the use of this API on Windows, redoing the DXGI screen capture in an external code.

Draft on top of !3226 (merged)