Track container touches in TouchManager

[calixteman]

Touch events are dispatched for the changed touches' targets, but evt.touches is global.

Track the identifiers that started on this manager's container and derive pinch state from that set. Unrelated fingers then neither keep editor pinch-resize alive nor become part of its pair.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.96970% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.05%. Comparing base (f43ce86) to head (d73dc99).
⚠️ Report is 11 commits behind head on master.

Files with missing lines	Patch %	Lines
src/display/touch_manager.js	96.96%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21722      +/-   ##
==========================================
+ Coverage   90.04%   90.05%   +0.01%     
==========================================
  Files         264      264              
  Lines       66935    66960      +25     
==========================================
+ Hits        60270    60303      +33     
+ Misses       6665     6657       -8     

Flag	Coverage Δ
browsertest	66.44% <0.00%> (-0.03%)	⬇️
fonttest	9.03% <ø> (+<0.01%)	⬆️
integrationtest	69.38% <84.84%> (+0.01%)	⬆️
unittest	58.13% <96.96%> (+0.16%)	⬆️
unittestcli	56.69% <0.00%> (-0.02%)	⬇️

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[timvandermeij]

r=me, with passing CI. Thanks!