memory: add user_data to GstMapInfo
memory: add user_data to GstMapInfo

Add extra pointers to GstMapInfo so that implementations can use these to store
extra info.

surround-io/gstreamer,jpxiong/gstreamer,justinjoy/gstreamer,mparis/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,centricular/gstreamer,lubosz/gstreamer,krieger-od/gstreamer,cablelabs/gstreamer,krichter722/gstreamer,cfoch/gstreamer,krieger-od/gstreamer,jpakkane/gstreamer,ensonic/gstreamer,krieger-od/gstreamer,lovebug356/gstreamer,cablelabs/gstreamer,centricular/gstreamer,cfoch/gstreamer,lovebug356/gstreamer,krichter722/gstreamer,krichter722/gstreamer,cablelabs/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,justinjoy/gstreamer,jpakkane/gstreamer,collects/gstreamer,ensonic/gstreamer,jpxiong/gstreamer,Lachann/gstreamer,justinjoy/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,krieger-od/gstreamer,cfoch/gstreamer,StreamUtils/gstreamer,lovebug356/gstreamer,mparis/gstreamer,mparis/gstreamer,jpakkane/gstreamer,collects/gstreamer,shelsonjava/gstreamer,lovebug356/gstreamer,surround-io/gstreamer,surround-io/gstreamer,centricular/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,ensonic/gstreamer,shelsonjava/gstreamer,jpakkane/gstreamer,collects/gstreamer,cfoch/gstreamer,lubosz/gstreamer,krichter722/gstreamer,collects/gstreamer,StreamUtils/gstreamer,cablelabs/gstreamer,StreamUtils/gstreamer,lubosz/gstreamer,surround-io/gstreamer,StreamUtils/gstreamer,justinjoy/gstreamer,lubosz/gstreamer,justinjoy/gstreamer,Lachann/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,ensonic/gstreamer,cablelabs/gstreamer,lovebug356/gstreamer,cfoch/gstreamer,Lachann/gstreamer,collects/gstreamer,jpakkane/gstreamer,Lachann/gstreamer,surround-io/gstreamer,Lachann/gstreamer,mparis/gstreamer,ensonic/gstreamer,jpxiong/gstreamer,lubosz/gstreamer,krieger-od/gstreamer