[Fix] Use opencv backend in Webcam API visualization

[Ben-Louis]

Motivation

The visualization step in Webcam API is slow due the matplotlib backend used by PoseVisualizer (#2035)

Modification

1. use imshow_bboxes and imshow_keypoints, which are used by Webcam API in 0.x version for visualization, to replace PoseVisualizer
2. rename webcam_demo to webcam_api_demo
3. add DeprecationWarning in demo/webcam_api_demo.py

BC-breaking (Optional)

Use cases (Optional)

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit tests to ensure correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Patch coverage: 40.42% and project coverage change: -0.43 ⚠️

Comparison is base (cd71020) 81.92% compared to head (c2fce8c) 81.49%.

❗ Current head c2fce8c differs from pull request most recent head 8f827f6. Consider uploading reports for the commit 8f827f6 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2089      +/-   ##
===========================================
- Coverage    81.92%   81.49%   -0.43%     
===========================================
  Files          227      227              
  Lines        13447    13513      +66     
  Branches      2286     2306      +20     
===========================================
- Hits         11017    11013       -4     
- Misses        1913     1980      +67     
- Partials       517      520       +3     

Flag	Coverage Δ
unittests	81.49% <40.42%> (-0.43%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...m/nodes/visualizer_nodes/object_visualizer_node.py	49.56% <40.42%> (-40.24%)	⬇️

... and 2 files with indirect coverage changes

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[ly015]

Please add a reference to the inferencer-based webcam demo.

[Ben-Louis]

In fact, the inferencer utilizing webcam input encounters a similar issue as the webcam API, where the visualization process is too slow to render keypoints in real time. As a result, users might have a less satisfactory experience with the inferencer compared to the updated webcam API.

[ly015]

Ok, I think this PR can be merged as it is.