We need to decide on a row vector vs. column vector convention;
while we can largely dodge this in documentation, that can
get pretty convoluted and doesn't make things easier for users.

This has an API effect ... the ordering of the arguments for
cairo_matrix_t is right for row vectors and wrong for column
vectors.

(My personal preference is for column vectors
 
 M*x = T*S*x

Correspnds to M(x) = T(S(x))

While x*M = x*S*T doesn't. The column vector convention I think
is also more common in math and marginally more common in 
computer graphics.)