[cairo] Fix a thinko in color glyph support

[Matthias Clasen]

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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