Demonstrate PDF Searchify() can put text in the wrong place

Check in a cleaned up copy of PDFium's bug_718762.in as big_image.in
and big_image.pdf. This PDF has a single 5000x5000 JPEG image that is
displayed as 64x64. The drastic difference between the actual image size
and rendered image size makes it easier to see when Searchify() is using
the wrong size value.

Then add a PDFiumEngineExportsTest to:

1) Load big_image.pdf
2) Confirm it has no text positions.
3) Searchify it.
4) Check the searchified PDF's text positions.

Note that the OCR results are on the page, yet the text positions have
negative origins.

Bug: 352482331
Change-Id: I2061e604f15c3c31d1ce333fff8789c7820ab95e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5697870
Reviewed-by: Chu-Hsuan Yang <chuhsuan@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1329337}