compositor: Move weston_matrix_transform_region to compositor.c and export it

We're going to use this to replace much of the other transform code so
it's no longer just relevant to pixman-renderer.c

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
[Pekka: add the warning about matrix restrictions]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/src/compositor.h b/src/compositor.h
index a07b51a..e64f30c 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -1646,6 +1646,10 @@
 			int32_t scale,
 			pixman_box32_t rect);
 void
+weston_matrix_transform_region(pixman_region32_t *dest,
+                               struct weston_matrix *matrix,
+                               pixman_region32_t *src);
+void
 weston_transformed_region(int width, int height,
 			  enum wl_output_transform transform,
 			  int32_t scale,