One of the changes, I'm not quite sure about: buildDisplay() used to open
and close the _thisScreen.background_layer_id resource for each layer it
processed. In particular, it used to "release the screen resource before
cacheing the sprites".
I have no idea why, because I can't see any trace of a sprite cache, and I
can't think of any harm in keeping the resource open during the whole
render cycle. The resource is probably loaded into memory already anyway,
though its reference counter may be 0.
svn-id: r13401