Compared to last attempt, I moved the format pointer to a dedicated `wg_transform_read_data` parameter instead of a property of the sample. Most of the transforms use `NULL` there as they don't expect or support format change. H264 decoder passes a transform format member where it tracks the current stream format.
-- v3: winegstreamer: Expose output media type attributes from the stream format. winegstreamer: Support wg_transform output format change events. winegstreamer: Avoid leaking buffer list in wg_transform_read_data. winegstreamer: Track caps changes and keep them with the output buffers. winegstreamer: Only filter video caps attributes when format explicitely uses 0. winegstreamer: Set wg_transform output sample size on gst_buffer_map failure. winegstreamer: Append a videoconvert element to wg_transform video output.