pdfinfo: improved paper size recognition

Paper sizes of ISO 216, A Series were originally defined in millimeters.
For example, A3 is defined to be 297mm x 420mm. However, depending on
source, the corresponding size in pts may vary between 1190pt and 1191pt
for the longer edge.

pdfinfo's formula to compute the length of the longer edge of an A3
paper determines this length to be 1191.82pt. As the error margin so far
was set to 1pt, A3 papers with edge length of 1190.8pt were not
recognized as A3.

This patch makes the error margin depending on the paper size, setting
it at 0.3% of the longer edge's length. For A3 paper, the error marging
(variable 'isoThreshold') thus becomes 3.58pt.

Accordingly, the threshold for 'letter' paper has been raised from 0.1pt
to 1pt.
6 jobs for pdfinfo-better-recognize-paper-format in 4 minutes and 26 seconds (queued for 19 seconds)
Status Job ID Name Coverage
  Build
passed #1064036
build

00:02:51

passed #1064041
build_android

00:01:04

passed #1064037
build_clang_libcpp

00:03:37

passed #1064040
build_clazy

00:02:09

passed #1064039
build_mingw64_fedora

00:02:35

passed #1064038
build_ubuntu_16_04

00:01:48