compositor: Disallow negative geometries in backend output configs

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Giulio Camuffo <giuliocamuffo@gmail.com>
diff --git a/src/compositor.h b/src/compositor.h
index 6bb6222..4443c72 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -619,9 +619,9 @@
  */
 struct weston_backend_output_config {
 	uint32_t transform;
-	int32_t width;
-	int32_t height;
-	int scale;
+	uint32_t width;
+	uint32_t height;
+	uint32_t scale;
 };
 
 /* Configuration struct for a backend.