Fix a thinko in composite_color_glyphs

We can't just move around the contents of the
passed-in string, we need to make a copy. This
was showing up as memory corruption in pango.

See https://gitlab.gnome.org/GNOME/pango/issues/346
1 job for master in 3 minutes and 44 seconds
Status Job ID Name Coverage
  Test
failed #431759
fedora:test

00:03:44

 
Name Stage Failure
failed
fedora:test Test
make[1]: *** [Makefile:906: check-recursive] Error 1
make[1]: Leaving directory '/builds/cairo/cairo'
make: *** [Makefile:1358: check] Error 2
Uploading artifacts...
test/*.log: found 2 matching files
test/pdiff/*.log: found 1 matching files

test/output: found 2231 matching files

Uploading artifacts to coordinator... ok
id=431759 responseStatus=201 Created token=Z9dgLyqc
ERROR: Job failed: exit code 1