Comment 7 for bug 1557893

In order to make the change build-able on Trusty, we need to add new interface into cairo.vapi:

--- vala-0.22-0.22.1.orig/vapi/cairo.vapi
+++ vala-0.22-0.22.1/vapi/cairo.vapi
@@ -645,6 +645,8 @@ namespace Cairo {
                public void mark_dirty_rectangle (int x, int y, int width, int height);
                public void set_device_offset (double x_offset, double y_offset);
                public void get_device_offset (out double x_offset, out double y_offset);
+ public void set_device_scale (double x_scale, double y_scale);
+ public void get_device_scale (out double x_scale, out double y_scale);
                public void set_fallback_resolution (double x_pixels_per_inch, double y_pixels_per_inch);
                public void show_page ();
                public Status status ();

Those two functions are indeed in Trusty's cairo, so I think it's safe to expose these interface for Vala.