width of propertized string

[David Reitter]

Given a propertized string, how would one find out what its (pixel)  
dimensions are going to be?

This is important to justify text and other graphical elements  
properly.  It is also necessary to correctly wrap (variable-width)  
text before displaying it.

I have asked about this a while ago, but I don't think anything has  
been implemented.  I'd be happy to give it a try, but I need pointers  
as regards which functions to use: I don't know the display engine at  
all.

I'd appreciate any help with this.