This creates a new libgstcodecs library that contains base classes to decode H264, H264 and VP9 with stateless accelerators. I need this split to introduce new elements for V4L2 StateLess. Note that it contains some improvement on top that I needed. Notably, it is now possible to use gst_video_decoder_get_frame() to retreive the frame as the system_frame_number is saved into the pictures now. I have ported D3D11 decoders to that, but it needs testing. The maximum DPB size is also passed to new_sequence() in H264 decoder, same will be needed for H265 but I'll do this slight API break in later commit (when I need it).