Support caching within and between QPicture instances

It would be nice if caching was supported for QPicure, both within the same instance and between instances.

For example, this could mean that a QPixmap could be painted to multiple QPictures and be shared between them. Then, when playing the QPictures to a PDF print engine, the pixmap would be cached in the generated PDF file as well, as the pixmap is the same across all QPictures, resulting in much smaller PDF files (imagine the difference in file size between 20+ page PDF files containing a 500 KB cached "water mark" image on each page vs. the same image on each page but not being cached).

Other painting resources, like brushes etc., could also be cached.