Merge lp://qastaging/~smspillaz/compiz-core/compiz-core.decor_synchronization into lp://qastaging/compiz-core/0.9.8

Add synchronization primitives to the decoration protocol so that there isn't a race where we bind a texture that's being freed.

Things changed

 * update_window_decoration_size made semi-private, users should use request_update_window_decoration_size instead, which prepares for a pixmap update and sends a message to the decorated process that a new pixmap is waiting.
 * decor_post_pending : tells the decorated process that a new decoration is pending
 * decor_post_request : tells the decorator that a new pixmap can be drawn to
 * decor_post_delete_pixmap : tells the decorator that a pixmap is no longer in use by the decorated process and can be deleted by the server

I also did some necessary refactoring so we could get this under test, namely:

DecorPixmap class (implements DecorPixmapInterface): a simple RAII class which ensures that a DecorPixmapDeletionInterface is invoked to delete the server side pixmap when the DecorPixmap class is destructed

X11DecorPixmapReceiver class (implements DecorPixmapReceiverInterface): a per-decoration owned class which tracks the update state of the pixmap owned by the decoration from the decorator, calls through to appropriate methods on a DecorPixmapRequestorInterface to request new pixmaps (ensuring that chattyness is reduced to when a) the decorator tells us that a new pixmap is ready and b) when a new pixmap was already pending and we need to fetch the newest one)

X11DecorPixmapRequestor class (implements DecorPixmapRequestorInterface): a per-window owned class which handles dispatch to the decorator (eg decor_post_pending) and requests from the decorator in _COMPIZ_DECOR_PENDING messages to communicate when decorations should be redrawn at new sizes

X11PixmapDeletor class (implements DecorPixmapDeletionInterface): a per-pixmap class which sends decor_post_delete_pixmap to the decorator when a pixmap is no longer in use by the decor plugin

DecorationListFindMatchingInterface: an interface implemented by DecorationList to find a decoration matching a specified criteria

DecorationInterface: an interface implemented by Decoration to get the receiver and properties about that decoration.

Tests included (Google Mock)