Enable CLUTTER / COGL_ENABLE_EXPERIMENTAL_API globally

Instead of defining CLUTTER_ENABLE_EXPERIMENTAL_API and
COGL_ENABLE_EXPERIMENTAL_API in individual source files, enable
them on the command line. We weren't tracking exactly what pieces of
experimental API we were using and we were using the experimental
API in most source files that used Clutter and Cogl, so the
local #defines were annoying rather than useful.

https://bugzilla.gnome.org/show_bug.cgi?id=685463