Inkscape takes a long time reading the gsub table of Calibri

Migrated from: https://bugs.launchpad.net/inkscape/+bug/1804860

Steps to reproduce:

open Inkscape and create a piece of text
change the font to Calibri (or other font with large gsub table)

What happened?

UI becomes unresponsive for up to 30 seconds

Comment from Patrick Storz (ede123) on original bug:
"I've seen this issue, too, also with other fonts. Simply selecting a different font in the font list (using text tool) takes a considerable amount of time (i.e. seconds).

This seems to be a fairly recent regression in master. 0.92.x built with identical library versions is unaffected, ruling out a regression with fontconfig / pango.

Also this seems not limited to Windows, Jabier reported experiencing this on Liunux, too... I suspect either OpenType font feature handling or the recent changes to SPStyle."

Inkscape Version and Operating System:

Inkscape Version: 1.0alpha (d7d7b92)
Operating System: Windows 10
Operating System version: 1803

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