add deprecation notice to mrlib/cache-image-snip
speed up tab switching

I belive this should only be noticeable at the human timescale level
when the tabs have been switched to already and, in that case, seems
to go from 3-400 msec to 50-60 msec on my machine

Essentially all of that time is in redrawing the buffer, so that 50-60
msec is proportional to the height of the DrRacket window, roughly.

(The commit just makes drracket do a single redraw instead of about 4
or so of them per switch ...)