[cairo] The right approach to projective transformations

[Bill Spitzak]

Behdad Esfahbod wrote:

> Right.  But we should be careful not adding yet another space.
> cairo_user_to_device_distance() will suffer no matter what.

I think this will have to return the device distance between the 
transforms of 0,0 and x,y.

Or maybe the current point and the current point plus x,y?

I am pretty certain that returning a dx,dy,dw value is useless. The 
delta of the w cannot be used to figure out any useful aspect of the 
transform.