Add a function that returns pixel distance between points?

[Yuan Fu]

For aligning text according to their display size, I need to calculate the 
width of some text. Currently I’m using window-text-pixel-size, but it has some 
problem: if the text I’m measuring has line-prefix or wrap-prefix, the size 
returned will include the width of the prefix. So if the prefix is 14 pixels 
wide and the text is 7 pixels wide, the returned value is 21 instead of 7. Line 
number width is sometimes included in the result and sometimes not.

There is also posn-at-point but that requires the point to be visible in the 
window. My alignment function wants to run in jit-lock so point is not always 
visible, so I can’t use posn-at-point.


Yuan