Fix a thinko in color glyph support

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I've spent some time tracking down a GTK+ / Pango crash (
https://gitlab.gnome.org/GNOME/pango/issues/346 ).

Turns out that there was a thinko in our original color glyph patch. We
can't modify the passed-in text.

Here is a quick fix:

https://github.com/matthiasclasen/cairo/commit/79ad01724161502e8d9d2bd384ff1f0174e5df6e

This probably needs to go into a stable cairo branch too.

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<div dir=3D"ltr"><div>I&#39;ve spent some time tracking down a GTK+ / Pango=
 crash ( <a href=3D"https://gitlab.gnome.org/GNOME/pango/issues/346">https:=
//gitlab.gnome.org/GNOME/pango/issues/346</a> ).</div><div><br></div><div>T=
urns out that there was a thinko in our original color glyph patch. We</div=
><div>can&#39;t modify the passed-in text.</div><div><br></div><div>Here is=
 a quick fix:<br></div><div><br></div><div><a href=3D"https://github.com/ma=
tthiasclasen/cairo/commit/79ad01724161502e8d9d2bd384ff1f0174e5df6e">https:/=
/github.com/matthiasclasen/cairo/commit/79ad01724161502e8d9d2bd384ff1f0174e=
5df6e</a></div><div><br></div><div>This probably needs to go into a stable =
cairo branch too.<br></div></div>

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