Support two-finger panning

[calixteman]

Pan by the midpoint delta and apply it with pinch zoom, keeping content under the fingers. Ignore non-cancelable moves, then re-baseline.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.31250% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.10%. Comparing base (fc30259) to head (5458a13).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
web/app.js	71.42%	2 Missing ⚠️
web/pdf_viewer.js	96.55%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21752   +/-   ##
=======================================
  Coverage   90.09%   90.10%           
=======================================
  Files         264      264           
  Lines       67237    67283   +46     
=======================================
+ Hits        60578    60624   +46     
  Misses       6659     6659           

Flag	Coverage Δ
browsertest	66.29% <0.00%> (-0.06%)	⬇️
fonttest	9.00% <ø> (ø)
integrationtest	69.31% <87.50%> (+0.01%)	⬆️
unittest	58.23% <49.12%> (+0.01%)	⬆️
unittestcli	56.77% <49.12%> (+0.19%)	⬆️

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[timvandermeij]

Thank you for implementing this!