Just like libmutter-clutter, and libmutter, mark exported symbols with
an COGL_EXPORT macro. This removes the .map and .map.in files previously
used, containing a list of semi private symbols. This symbol was out of
date, i.e. pointed to non-existing symbols, and was also replaced with
COGL_EXPORT macros.
unit_test_* symbols are exported by the help of the unit test defining
macro. test_* symbols are no longer supported as it proved unnecessary.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/1059
This macro was introduced so as to be able to be built without GLib.
However, this feature was long ago removed, and in Mutter we depend on
it anyway, so let's get rid of it in favor of more consistency.
