Executing Key Sequences directly

[Steven Tamm <steventamm@mac.com>]

While working on the Mac OS X carbon port I was trying to switch from 
using a "polling" style of event handling to a "callback" style of 
event handling.  The intent is that the UI will run in a different 
thread from the eval loop, in a way similar to the w32 port.   However 
the port also needs to work so that it can work if the UI and the eval 
loop are in the same thread.  The complication comes from the scroll 
bars; the way that Mac OS X handles scroll bars is through a callback 
while in an "event loop" for the scroll bar.  So while doing live 
scrolling, the command loop never gets re-entered because it's stuck in 
another loop.  What is the best way to have this callback evaluate the 
lisp code for a specific keymap (in this case <vertical-scroll-bar> 
<mouse-down>)?   Should I use read_key_sequence?

Previously, the port didn't use this mechanism at all, but instead 
trapped all mouse down events and passed them to the system.  I'm 
trying to avoid this, but if there is no "good" way to evaluate a 
key-sequence inside C-code, this is what it will continue to do.

Thanks,
-Steven