[cairo] rendering changes cairo 1.16 -> 1.17.4

[Felix Schwarz]

Hi,

Am 28.02.21 um 23:32 schrieb Heiko Lewin:
> You are referring to commits dealing with glyph-rasterization. The
> images you posted do not really seem to be concerned with glyphs at all.

I digged more into this and actually the test failure is related to glyphs even 
though it was not obvious from the PNGs I provided.

The test use a special ttf font named "ahem": 
https://web-platform-tests.org/writing-tests/ahem.html

Step-by-step I inlined more WeasyPrint code (and deleted seemingly unnecessary 
branches and special cases) and came up with this:
https://github.com/FelixSchwarz/cairo117-repro/blob/main/minimal.py

(Yes, it's far from what you probably consider "minimal" but at least you can 
see some cairo interaction.)

The repo also contains some PNGs with the output on 1.16 and the output on 1.17:
https://github.com/FelixSchwarz/cairo117-repro


Felix