Future of shadow and shadowfb

[Adam Jackson]

Did you know we have two of these?  Can we have only one of these?

I'd like to merge them together, one way or another, but there are some 
slight semantic differences.  shadowfb gives you hooks for both pre- and 
post-rendering callbacks.  Exactly one driver uses this, vmware, for 
what looks like tearing down and putting back up the cursor.  I'm 
reasonably sure that could be better handled, but it's also easy enough 
to preserve.

More interestingly, shadow batches updates until BlockHandler, whereas 
shadowfb is immediate.  I would expect shadow's performance to be better 
in the typical use case since it would reduce overdraw.  I don't know of 
any drivers that _depend_ on shadowfb giving an immediate callback, but 
conversely BlockHandler isn't exactly a great interface for periodic 
updates, since it gets called on no particular schedule.

So my thinking is:

- port shadowfb to damage instead of handrolling it
- merge that with shadow since they'll then be largely equivalent
- keep the external API intact since it's easy enough
- hack loader to alias 'shadow' and 'shadowfb' together

Anything I'm missing here?

- ajax