winegstreamer: Only add the capsfilter for avdec_h264.
avdec_h264 will fail when combined with h264parse and a drain request is made during caps neg.
To fix this, a capsfilter was added to force a stream-format of byte-stream for all h.264 decoders. However, this introduced a new issue with the vtdec element, as it only supports the avc stream-format.
Therefore, this patch looks to address both problems by only introducing the capfilter when the avdec_h264 element is used.