Feature request: dimensions of propertized string

[David Reitter]

Can I make a request for a feature here?

I'd like a function hat, given a propertized string and a frame, tells  
me what the dimensions (in particular pixel width) of that string are  
if it was rendered in that frame.

I've needed this several times when trying to figure out where best to  
wrap text when proportional fonts are used.

A few days ago I asked for a "font-variable-width-p" function in  
gnu.emacs.help and didn't get a reply, and I think such a function can  
be constructed easily once I know the dimensions of a propertized  
string.



===

I'm trying to figure out whether a given face (the default face in a
frame, to be precise) uses a variable width font, but I can't find an
appropriate function in Emacs.
One could compare the pixel widths of 'w' and 'i', but I can't find a
function that would give me the pixel width of a (propertized) string,
either.

Any suggestions?