Incorporate italic sources into build about shantell-sans

[arrowtype]

Incompatibilities to fix, Jun 9:

ERROR:cu2qu.ufo:Glyphs named 'arrowNE' have different number of segments
ERROR:cu2qu.ufo:Glyphs named 'arrowNW' have different number of segments
ERROR:cu2qu.ufo:Glyphs named 'arrowSW' have different number of segments
ERROR:cu2qu.ufo:Glyphs named 'arrowSE' have different number of segments
ERROR:cu2qu.ufo:Glyphs named 'invertedbrevecmb' have different number of segments
ERROR:cu2qu.ufo:Glyphs named 'bracketangleright' have different number of segments
ERROR:cu2qu.ufo:Glyphs named 'dotbelowcmb' have different number of segments
ERROR:cu2qu.ufo:Glyphs named 'caroncomb' have different number of segments

Solution: add these as glyphs to decompose in prep-build.py. Very likely, they were getting decomposed in select sources only, by the ufo2ft filter that decomposes scaled/flipped components.

[arrowtype]

Okay! It's a subtle but important difference: the bouncy italics now have their yShift compensated for with an xShift.

In the below video, the old font is the bottom row, and the new font is the top row. The old font moves only vertically, causing problems in the bouncy versions of glyphs, while the new font moves along the italic slant, allowing for better spacing.

Screen.Recording.2022-06-09.at.5.29.16.PM.mov

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