[BUG] nvdsvideotemplate's customlib-props can't be retrieved

[CristiFati]

Notes:

• This is not a bug, but an enhancement
• Implementing it would not have a (direct) functional benefit on videotemplate
• Might apply to other elements

Describe the bug
I created a tool (pycfgst, CristiFati/pycfgst) that converts a programmatically created pipeline (from Python) into its gst-launch-1.0 equivalent (of course considering all the latter's limitations).

It does that by identifying the elements, links between them and for each element getting its properties and their values and filtering the ones that are not needed / relevant.

The problem is that for elements like nvdsvideotemplate that is not possible, because when setting customlib-props multiple times (a very common scenario) only the last one is possible to be retrieved back

Steps/Code to reproduce bug
Running the PipelineParserNVidiaTestCase.test_videotemplate_original_separate from CristiFati/pycfgst - test_gstreamer.py yields:

[root@cfati-desktop:/mnt/work/github/cfati/pycfgst/src]> python -m unittest pycfgst.tests.test_gstreamer.PipelineParserNVidiaTestCase.test_videotemplate_original_separate
------- Custom0::SetProperty: [prop0] = [val0]
------- Custom0::SetProperty: [prop1] = [val1[{]},<.>/?|;:]
------- Custom0::SetProperty: [prop2] = [val 2 ]
Output:
gst-launch-1.0 -e \
  videotestsrc \
      num-buffers=1 \
      pattern=18 \
  ! nvvideoconvert \
      src-crop=0:0:0:0 \
      dest-crop=0:0:0:0 \
  ! nvdsvideotemplate \
      customlib-name=/mnt/work/github/cfati/pycfgst/src/pycfgst/tests/data/nvdsvideotemplate/testlib0/libvttest0.so \
      customlib-props='prop2:val 2 ' \
  ! fakevideosink \
      max-lateness=20000000
.
----------------------------------------------------------------------
Ran 1 test in 0.408s

OK

Although properties are correctly propagated to the library, only the last one is got back.

Expected behavior
The output pipeline command string should contain all 3 properties.

Environment details (please complete the following information):
N/A - tried in a Ubuntu DS container

Additional context
although I know DeepStream is not accepting any contributions, I took the liberty of creating a patch (nvdsvideotemplate-ds90-sep_del.patch) that fixes this issue:

• Allows setting multiple library properties via one customlib-props specification
• Adds a customlib-props-sep property to nvdsvideotemplate that is used to separate library properties
• Only affects the "old" way of specifying properties (prior to config file)
• Also fixes [BUG] Undefined behavior in nvdsvideotemplate handling custom library properties string #64

Running the test_videotemplate_original_joined test (from above) with and without the patch applied (in the 2nd run, the properties are correctly passed to the custom library):

[root@cfati-desktop:/mnt/work/github/cfati/pycfgst/src]> python -m unittest pycfgst.tests.test_gstreamer.PipelineParserNVidiaTestCase.test_videotemplate_original_joined
videotemplate does NOT support customlib-props-sep, 1 line(s) from custom library should be displayed
------- Custom0::SetProperty: [prop0] = [val0;;;prop1:val1[{]},<.>/?|;:;;;prop2:val 2 ]
Output:
gst-launch-1.0 -e \
  videotestsrc \
      num-buffers=1 \
      pattern=18 \
  ! nvvideoconvert \
      src-crop=0:0:0:0 \
      dest-crop=0:0:0:0 \
  ! nvdsvideotemplate \
      customlib-name=/mnt/work/github/cfati/pycfgst/src/pycfgst/tests/data/nvdsvideotemplate/testlib0/libvttest0.so \
      customlib-props='prop0:val0;;;prop1:val1[{]},<.>/?|;:;;;prop2:val 2 ' \
  ! fakevideosink \
      max-lateness=20000000
.
----------------------------------------------------------------------
Ran 1 test in 0.483s

OK

[root@cfati-desktop:/mnt/work/github/cfati/pycfgst/src]> python -m unittest pycfgst.tests.test_gstreamer.PipelineParserNVidiaTestCase.test_videotemplate_original_joined
videotemplate DOES support customlib-props-sep, 3 line(s) from custom library should be displayed
------- Custom0::SetProperty: [prop0] = [val0]
------- Custom0::SetProperty: [prop1] = [val1[{]},<.>/?|;:]
------- Custom0::SetProperty: [prop2] = [val 2 ]
Output:
gst-launch-1.0 -e \
  videotestsrc \
      num-buffers=1 \
      pattern=18 \
  ! nvvideoconvert \
      src-crop=0:0:0:0 \
      dest-crop=0:0:0:0 \
  ! nvdsvideotemplate \
      customlib-name=/mnt/work/github/cfati/pycfgst/src/pycfgst/tests/data/nvdsvideotemplate/testlib0/libvttest0.so \
      customlib-props='prop0:val0;;;prop1:val1[{]},<.>/?|;:;;;prop2:val 2 ' \
      customlib-props-sep=';;;' \
  ! fakevideosink \
      max-lateness=20000000
.
----------------------------------------------------------------------
Ran 1 test in 0.423s

OK

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