compositor: Move repaint debug binding to mod-shift-space

Alt-space or Super-space are too likely to conflict with application
bindings.
diff --git a/src/compositor.h b/src/compositor.h
index a144e5f..0d49dec 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -52,6 +52,7 @@
 	MODIFIER_CTRL = (1 << 0),
 	MODIFIER_ALT = (1 << 1),
 	MODIFIER_SUPER = (1 << 2),
+	MODIFIER_SHIFT = (1 << 3),
 };
 
 enum weston_led {