Generic matrix operations.

cairo::matrix_t is used throughout cairo to convert between different coordinate
spaces. A [[cairo::matrix_t]] holds an affine transformation, such as a scale,
rotation, shear, or a combination of these. The transformation of a point (x,y)
is given by:

    x_new = xx * x + xy * y + x0;
    y_new = yx * x + yy * y + y0;

The current transformation matrix of a [[cairo::context_t]], represented as a
[[cairo::matrix_t]], defines the transformation from user-space coordinates to
device-space coordinates. See [[cairo::get_matrix]] and [[cairo::set_matrix]].
