Fix recorder overriding tracks instead of merging on add_tracks. Release 0.8.1

[sgfn]

Fixes issue outlined in #194

The new tracks were correctly merged into state in line 169 but state was then erroneously updated again in line 180, causing the freshly added tracks to override the existing ones.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.13%. Comparing base (12d9b04) to head (acae32b).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ex_webrtc/recorder.ex	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #195      +/-   ##
==========================================
- Coverage   85.25%   85.13%   -0.12%     
==========================================
  Files          49       49              
  Lines        2584     2584              
==========================================
- Hits         2203     2200       -3     
- Misses        381      384       +3     

Files with missing lines	Coverage Δ
lib/ex_webrtc/recorder.ex	0.00% <0.00%> (ø)

... and 2 files with indirect coverage changes

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