[cairo] Adding cairo_shear and family

[Bill Spitzak]

It might make more sense to make the call do the composite operation 
that preserves area:

[1+sx*sy sx]
[sy 1+sx*sy]

Note that if either sx or sy are zero then it is the same as the 
proposed call.

I do not think specifying the shear as an angle is useful or desirable.

The main reason for shear is because an arbitrary matrix can be 
decomposed into rotate, scale, and shear (in one direction only). This 
is necessary to copy a matrix to another api which lacks any direct 
ability to set the matrix.