libvlc: add libvlc_video_engine_anw support
On top of !7026 (merged)
This allows to render MediaCodec output, or CPU output in Vulkan (leaving all Vulkan specific code in the external app), cf the Vulkan sample here: https://code.videolan.org/tguillem/libvlc-android-samples/-/commits/vulkan
This could also be used with onscreen SurfaceViews, bypassing the need of creating an intermediate Java object (AWindowHandler.java).