layer: Set clip start to the duration of the layer if == TIME_NONE
layer: Set clip start to the duration of the layer if == TIME_NONE

In the provided start of a clip is GST_CLOCK_TIME_NONE in
ges_layer_add_asset, it means that we want the clip to be
added at the end of the layer

cfoch/ges,cfoch/ges,cfoch/ges,cfoch/gst-editing-services,cfoch/ges,cfoch/gst-editing-services,cfoch/gst-editing-services,cfoch/gst-editing-services