Poppler/cairo import incorrectly transforms transparent gradients in pdf created with cairo > 1.11.4

Reported in https://bugs.launchpad.net/inkscape/+bug/1078430 , highly related to #3281 (moved) (internal import).

Summary:

• poppler/cairo import incorrectly converts transparent gradients of PDFs to SVG (created by Inkscape export, cairo >= 1.11.4, saving as pdf or printing to pdf via GTK+ print dialog)

Steps to reproduce:

• open Inkscape
• open this svg or create a rectangle and give it a gradient (should be #0000ffff to #0000ff00 by default)
• save as pdf (1.4 or 1.5, default values)
• open created pdf in Inkscape (poppler/cairo import)

What happened?

• The gradient fills are displaced and partially clipped off
• If you look at the radial gradient, it is the proper size (see the mask), but the transform stretches the gradient into the wrong shape.

What should have happened?

• The gradient fills in original SVG and opened PDF look identical (or at least very similar)

Pdfs affected

Affects newer versions of cairo

• problematic pdfs created (by su_v) with cairo 1.12.4, 1.12.6 and 1.12.8 on OS X 10.7.4 (64bit) with GTK+/Quartz builds of Inkscape 0.48.3.1 and Inkscape 0.48+devel r11871.
• unproblematic pdfs created (by su_v) with cairo (all <= 1.12.2) 1.8.8, 1.8.10 and 1.10.2 on all official Mac OS X Inkscape packages >= 0.47 (in 2012-11-13)
• unproblematic pdfs created (by su_v) with cairo 1.10.2 (X11), 1.11.4 (Quartz) with local builds on Mac OS X 10.5.8 (32bit)
• rsvg-convert pdfs

pdf rendered in pdf viewers

Problematic pdf (created with cairo 1.12.8)

• crashed/segfaulted xpdf 3.03 on Ubuntu (does not segfault 3.04-7 on Linux Mint 19.1).
• rendered with solid fill colors instead of gradient fills on Ghostscript 9.05 on OS X
• rendered normally on Evince 2.30.3 (OS X), 3.6.0 (Ubuntu 12.10), Xpdf 3.02 (OS X), Ghostscript 9.06 (Ubuntu 12.10) and Apple's Preview.app (OS X)

Doesn't affect

• pdfs created with libcairo 1.11.4 or older
  • unproblematic pdfs created (by su_v) with cairo 1.12.2 (X11) with local builds on OS X 10.7.4 (64bit)
  • unproblematic pdfs created (by su_v) with cairo 1.12.2 on a Ubuntu 12.10 x64 VM
• fully opaque gradients, but these imports in a completely different way (transparent gradients are separated into opaque gradient and a mask with a transparency gradient)

Example files

• 04831-1.svg - linked above
• 04831-1-cairo-1_15_10-2ubuntu0_1-838ae0f22d-1_1-dev.pdf - Inkscape 1.1-dev (838ae0f22d, 2020-07-14, custom) export, looks slightly different from others
• 04831-1-cairo-from-7z-0_48_5-1_windows.pdf - should be correct, created with 0.48.5-1 r10040 x32 7z archive on Windows 10 x64
• 04831-1-cairo-1_12_8-04831.pdf - PDF exported with Inkscape 0.48.3.1 and cairo 1.12.8
• 04831-1-cairo-1_12_8-r11871.pdf - PDF exported with Inkscape 0.48+devel r11871 and cairo 1.12.8
• 04831-1-cairo-1_12_8-librsvg-2_36_4.pdf - should be imported correctly, but pdf is a different size
• 04831-1-cairo-1_12_8-librsvg-2_36_4-zoom_0.8.pdf - linked above

Version Info:

• Reproduced Inkscape 1.1-dev (838ae0f22d, 2020-07-14, custom) Linux Mint 19.1, libcairo2 1.15.10-2ubuntu0.1
• Reproduced Inkscape 0.92.5 (6c0c36ef5a, 2020-05-15) Linux Mint 19.1, libcairo2 1.15.10-2ubuntu0.1
• Reproduced Inkscape 0.91 r13725 on Kubuntu (from launchpad)
• Reproduced Inkscape trunk r11871 on Mac OS X 10.5.8 (32bit), OS X 10.7.4 (64bit) and Ubuntu 12.10 (64bit, VM) (from launchpad)