cairo: Add complete tests for cairo.Matrix

Signed-off-by: Uli Schlachter <psychon@znc.in>
cairo: Remove extents() method from cairo.Path

This calls the C function cairo_path_extents(). However, this function expects a
cairo context (cairo_t*) as its first argument and returns the extents of the
current path. This function is already correctly described for cairo.Context.

cairo_path_t is just a public struct without any methods.

Signed-off-by: Uli Schlachter <psychon@znc.in>
cairo: Add new properties to cairo contexts

This adds the group_target, matrix and font_matrix properties to cairo contexts.

This uses cairo_get_group_target() which is already handled correctly. This new
group_target property complements the existing target property, which is
read-only, too.

Similar, the setters and getters for matrix and font_matrix already exist and
can easily be turned into a property

Signed-off-by: Uli Schlachter <psychon@znc.in>
cairo: Add tests for all context properties

Signed-off-by: Uli Schlachter <psychon@znc.in>
Merge pull request #63 from psychon/more-cairo-tests

Some more cairo tests (and small fixes/improvements)