[plt-scheme] MrEd: get-text-extent in dc<%>

I notice that get-text-extent is on dc<%>, not font%.  Is this
guaranteed to return the same extent for a given string and font on
a bitmap-dc% and the dc of a canvas?  More generally, will drawing to
a bitmap and copying to a canvas always look the same as drawing
directly to the canvas?

Also, why does get-text-extent on a bitmap-dc% complain if the bitmap
is #f?  I have a chicken-and-egg problem-- I want to draw some text
on a bitmap, but I don't know how big to make the bitmap, but I can't
call get-text-extent until I make a bitmap.  It works if I make a 1x1
bitmap and then replace it with the real one, but that seems like a
hack...

--dougo at ccs.neu.edu