[cairo] Adding cairo_shear and family

[Behdad Esfahbod]

Heya,

This is a small API addition I've wanted to make for a while.  I propose
adding cairo_shear (cairo_t *cr, double sx, double sy) and respective
cairo_matrix_t versions to cairo.  These will multiply by [1 sx sy 1 0 0].
The reason for this is that with the shear operation, any matrix can be built
using the provided operations (translate, rotate, scale, shear), while
currently one has to resort to a full-matrix transform for some matrices.

Fontconfig has this btw.

Cheers,

behdad