This method will render progress bars and spinners, these should
handle active and inconsistent states' animation progress in order
to perform animations.
It actually messed up with the state being actually set, and having a
transition animation running for it. Now this dichotomy is removed, and
gtk_style_context_state_is_running() only checks whether an animation is
running, leaving state checking to flags & GTK_STATE_FLAG_*.
The case for paned can be guessed out from the height and width,
for handlebox the orientation parameter doesn't make much sense,
and this way it could also be used for resize grips.
This support goes from the theming engines, which are able to retrieve
style for different combined states to the CSS provider, where several
state pseudo-classes may be specified, such as:
GtkButton:active:prelight {}
This function may be used for custom property registration from
theming engines. The property names will have the
-${engine-type-name}-${prop-name} format, the parser has been
modified to allow properties with '-' as the first char.
GtkThemingEngine will be the theming engines base class, with default
implementations for all paint functions, and readonly access to the
related GtkStyleContext data.
