distribute-made copy of screens.rpy.
Fix some issues with edgescroll.

* It could start on timerevent, rather than a mouse event.
* It could get the wrong edge_last_st, leading to large jumps.
Try harder to find a place to return to.

When the return site exists, we simply return to it. If the return
site disappears, we look up the call site, and return to the next
node, if one exists.

If we're in developer mode, we report an error at this
point. Otherwise, we pop the stack, and try again.
Add alt text to imagemap hotspots.
Update docs.
Add a script that can compare zip sizes.

This lets us attribute changes in size to particular files.