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,
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 {
diff --git a/src/shell.c b/src/shell.c
index 2e1e22a..2222970 100644
--- a/src/shell.c
+++ b/src/shell.c
@@ -3248,7 +3248,7 @@
 					  ec);
 	weston_compositor_add_key_binding(ec, KEY_BRIGHTNESSUP, 0,
 				          backlight_binding, ec);
-	weston_compositor_add_key_binding(ec, KEY_SPACE, mod,
+	weston_compositor_add_key_binding(ec, KEY_SPACE, mod | MODIFIER_SHIFT,
 				          debug_repaint_binding, shell);
 	weston_compositor_add_key_binding(ec, KEY_K, mod,
 				          force_kill_binding, shell);