screen: Add screen.by_coords()

This idea came from here:
http://article.gmane.org/gmane.comp.window-managers.awesome.devel/6759

Signed-off-by: Uli Schlachter <psychon@znc.in>

screen.c

@@ -662,9 +662,26 @@ luaA_screen_count(lua_State *L)
     return 1;
 }
 
+/** Get a screen by coordinates.
+ * \param L The Lua VM state.
+ * \return The number of elements pushed on stack.
+ *
+ * \luastack
+ * \lreturn The screen count, at least 1.
+ */
+static int
+luaA_screen_by_coords(lua_State *L)
+{
+    int x = luaL_checknumber(L, 1);
+    int y = luaL_checknumber(L, 2);
+    luaA_pushscreen(L, screen_getbycoord(x, y));
+    return 1;
+}
+
 const struct luaL_reg awesome_screen_methods[] =
 {
     { "count", luaA_screen_count },
+    { "by_coords", luaA_screen_by_coords },
     { "__index", luaA_screen_module_index },
     { NULL, NULL }
 };