Create a demo/API to access shadow state of the tracker

[bwsw]

The docs:
https://docs.nvidia.com/metropolis/deepstream/dev-guide/text/DS_plugin_gst-nvtracker.html#shadow-tracking-target-data

[ara-25]

Hey @bwsw
I have used this some while ago (used Savant v0.4.5, DS 6.4). At least for DS 6.4, the python bindings to access shadow tracking data were not working correctly. To get them to work, I had to recompile DS python with some changes, according to this: https://forums.developer.nvidia.com/t/how-to-fetch-nvdstracker-shadow-tracking-data-using-python/298397/3

[ara-25]

Also, I am willing to contribute to this if its not assigned to anyone else as of now

[bwsw]

@ara-25 it would be perfect!