compositor, shell: add and use weston_surface_set_position()

Another shorthand like weston_surface_configure().

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
diff --git a/src/compositor.h b/src/compositor.h
index 1b0d5cb..966d3f4 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -399,6 +399,10 @@
 			 GLfloat x, GLfloat y, int width, int height);
 
 void
+weston_surface_set_position(struct weston_surface *surface,
+			    GLfloat x, GLfloat y);
+
+void
 weston_surface_assign_output(struct weston_surface *surface);
 
 void