exporting to png produces wrong colours on transparent pixels

Note to self: investigate whether related to or same as
* Bug #882236 “Antialiasing changes shape color when exported to image file”
  https://bugs.launchpad.net/inkscape/+bug/882236

Note to toxigames: black color of fully transparent pixels is tracked here:
* Bug #1430716 “Bug with export-background color if opacity=0”
  https://bugs.launchpad.net/inkscape/+bug/1430716

Testing with GIMP layer alpha threshold set to 0 (see bug #882236), it is to be noted that opening the sample SVG file itself in GIMP (2.8.16, 2.9.3) shows the same unexpected colors for the (anti-aliased transparent) pixels as with Inkscape's bitmap export (0.91, 0.92) - it seems that this is not a unique issue to be fixed in Inkscape's own renderer.

Hi su_v, I just opened the example svg (that were created in Inkscape) in Illustrator CS6 and exported a png. The unexpected colors are not present there, the PNG has only one color. I did the same in GIMP and Inkscape, and both does indeed still create the unexpected colors on the transparent antialiased edge. I wonder why Illustrator is working properly with the PNG export, while Inkscape and GIMP does not?

Confirming for now as is - the reported issue reproduces in rasterized output based on the same SVG file with Inkscape 0.91 r13725, 0.92.0 r15299, GIMP 2.9.3, and rsvg-convert (librsvg 2.40.13) on OS X 10.7.5, with cairo 1.14.6 and 1.15.5 (git).

Likely, this is the same (or a closely related) issue as already tracked in bug #882236 - the test case provided here though does not expose the same obvious symptoms with cli exports using Inkscape 0.48.5 (unlike bug #882236, and unlike as experienced by the affected user (see forum postings)).

Made a test image with lowered opacities, fading gradients, lowered alphas, compositing with filters, masking and clipping. All seems affected by the issue in win64 Inkscape 0.92.0 r15299.