Fix "Scanning contents" progress bar with --redo-ocr

[aliemjay]

A followup fix to #1378.

The progress bar stalls at 0% until all pages are parsed and cached, which takes pretty much of the total run time (thanks to #1380). Fix this by lazy caching.

Test file: blank-10k.pdf
Test command: ocrmypdf --redo-ocr blank-10k.pdf out.pdf

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.19%. Comparing base (cc058be) to head (6d2d870).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ocrmypdf/pdfinfo/layout.py	77.77%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1441   +/-   ##
=======================================
  Coverage   90.18%   90.19%           
=======================================
  Files          95       95           
  Lines        7073     7070    -3     
  Branches      722      720    -2     
=======================================
- Hits         6379     6377    -2     
+ Misses        491      490    -1     
  Partials      203      203           

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