glib: Make PrintScaling preference available in API

Add poppler_document_get_print_scaling() function and
PopplerPrintScaling enum so that applications which
use poppler's glib frontend can access this preference.

https://bugs.freedesktop.org/show_bug.cgi?id=92779
6 jobs for glib-export-printscaling-preference in 12 minutes and 22 seconds (queued for 57 seconds)
Status Job ID Name Coverage
  Build
passed #74811
build

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passed #74816
build_android

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passed #74812
build_clang_libcpp

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build_clazy

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build_gcc_49

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passed #74814
build_mingw64_fedora

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