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.c b/src/compositor.c
index 8cf66bb..df39274 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -1798,6 +1798,8 @@
 		seat->modifier_state |= MODIFIER_ALT;
 	if (mods_lookup & (1 << seat->xkb_info.super_mod))
 		seat->modifier_state |= MODIFIER_SUPER;
+	if (mods_lookup & (1 << seat->xkb_info.shift_mod))
+		seat->modifier_state |= MODIFIER_SHIFT;
 
 	/* Finally, notify the compositor that LEDs have changed. */
 	if (xkb_state_led_index_is_active(seat->xkb_state.state,