cogl-pango is conceptually a separate library so it doesn't seem
appropriate to bundle the headers with all the other cogl headers. Also
in-tree the headers live in a cogl-pango directory so if we want
examples that can include cogl-pango consistently when built in or out
of tree using the convention #include <cogl-pango/cogl-pango.h> makes
that easy.
This adds a compatibility cogl/cogl-pango.h header that's will redirect
to cogl-pango/cogl-pango.h with a warning, or result in an error if
COGL_ENABLE_EXPERIMENTAL_2_0_API is defined.
