exposay: set an alpha on fullscreen surfaces

This makes the desktop background actually draw when there is
a fullscreen surface and we go to exposay.
diff --git a/desktop-shell/exposay.c b/desktop-shell/exposay.c
index f409aa3..01bf0b1 100644
--- a/desktop-shell/exposay.c
+++ b/desktop-shell/exposay.c
@@ -286,6 +286,8 @@
 		if (shell->exposay.focus_current == esurface->view)
 			highlight = esurface;
 
+		set_alpha_if_fullscreen(get_shell_surface(view->surface));
+
 		exposay_animate_in(esurface);
 
 		i++;
diff --git a/desktop-shell/shell.c b/desktop-shell/shell.c
index f85a269..4cc23d5 100644
--- a/desktop-shell/shell.c
+++ b/desktop-shell/shell.c
@@ -222,6 +222,13 @@
 	} popup_grab;
 };
 
+void
+set_alpha_if_fullscreen(struct shell_surface *shsurf)
+{
+	if (shsurf && shsurf->state.fullscreen)
+		shsurf->fullscreen.black_view->alpha = 0.25;
+}
+
 static struct desktop_shell *
 shell_surface_get_shell(struct shell_surface *shsurf);
 
diff --git a/desktop-shell/shell.h b/desktop-shell/shell.h
index 7a8194d..8ef550f 100644
--- a/desktop-shell/shell.h
+++ b/desktop-shell/shell.h
@@ -188,6 +188,9 @@
 	char *client;
 };
 
+void
+set_alpha_if_fullscreen(struct shell_surface *shsurf);
+
 struct weston_output *
 get_default_output(struct weston_compositor *compositor);