Only send PLIs for video tracks in examples

[tmatth]

Refs pion/example-webrtc-applications#228

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.85%. Comparing base (ddb9219) to head (6be352e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2876      +/-   ##
==========================================
- Coverage   78.95%   78.85%   -0.10%     
==========================================
  Files          89       89              
  Lines        8452     8452              
==========================================
- Hits         6673     6665       -8     
- Misses       1294     1299       +5     
- Partials      485      488       +3     

Flag	Coverage Δ
go	80.41% <ø> (-0.11%)	⬇️
wasm	65.11% <ø> (ø)

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