Avoid clutter segfaulting when used without invoking
clutter_init(). This is needed when using api documentation
tools and every other tool relying on the GObject
introspection API (esp. on a headless box). see the
note in clutter/clutter-feature.c:clutter_feature_init
for a full explanation.
* clutter/clutter-feature.c: Call clutter_feature_init()
when needed by one of the accessors of the features
structure.
* clutter/clutter-main.c:
* clutter/clutter-private.h: Remove clutter_feature_init()
public declaration: the features support check is done the
first time a feature is needed.
* clutter/clutter-main.c: Do not ever access the clutter
main context pointer directly; instead, obtain a pointer
to it via clutter_context_get_default(), which will always
return something valid.