Release the screenPixmap as soon as painting is over, so that the associated shared buffer can be reused.

 * a new method releaseScreenPixmap has been added to the Window class
 * it is called in Window.cpp and CardWindow.cpp when painting is done
 * in HostWindowDataOpenGLHybris, the cached texture is released as soon as releaseScreenPixmap is called
 * also, see bug http://issues.webos-ports.org/issues/393

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>