Use pixel coordinates for weston_output.matrix

Previously, weston_output.matrix was in GL coordinates and therefore only
really useful for the GL backend.

This breaks zoom, which will be fixed by the following patch:
	zoom: Use pixels instead of GL coordinates

[Pekka: added a comment to compositor.h, message]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-By: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
diff --git a/src/compositor.h b/src/compositor.h
index bb7562f..207767e 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -182,7 +182,10 @@
 	struct wl_list resource_list;
 	struct wl_global *global;
 	struct weston_compositor *compositor;
+
+	/** From global to output buffer coordinates. */
 	struct weston_matrix matrix;
+
 	struct wl_list animation_list;
 	int32_t x, y, width, height;
 	int32_t mm_width, mm_height;