image: add alpha property

Signed-off-by: Julien Danjou <julien@danjou.info>

common/tokenize.gperf

@@ -1,4 +1,5 @@
 align
+alpha
 bar_data_add
 bar_properties_set
 bg

image.c

@@ -358,6 +358,10 @@ luaA_image_index(lua_State *L)
         imlib_context_set_image((*image)->image);
         lua_pushnumber(L, imlib_image_get_height());
         break;
+      case A_TK_ALPHA:
+        imlib_context_set_image((*image)->image);
+        lua_pushboolean(L, imlib_image_has_alpha());
+        break;
       default:
         return 0;
     }