[lua] We cannot detect existent keys with nil in a table (they aren't added to the table)

All non-existent keys return nil (and nil keys are not added when the
table is created, so we cannot even iterate the table to search for
keys like "always" or "sRGB"). The only solution is assigning a
non-nil value, e.g. { always=true }, { sRGB=true }, etc.

src/app/script/color_space_class.cpp

@@ -40,7 +40,7 @@ int ColorSpace_new(lua_State* L)
       lua_pop(L, 1);
 
     // Create sRGB profile with ColorSpace{ sRGB }
-    if (lua_getfield(L, 1, "sRGB") != LUA_TNONE) {
+    if (lua_is_key_true(L, 1, "sRGB")) {
       lua_pop(L, 1);
       push_new<gfx::ColorSpace>(L, *gfx::ColorSpace::MakeSRGB());
       return 1;

src/app/script/dialog_class.cpp

@@ -379,14 +379,9 @@ int Dialog_newrow(lua_State* L)
   dlg->autoNewRow = false;
   if (lua_istable(L, 2)) {
     // Dialog:newrow{ always }
-    const int type = lua_getfield(L, 2, "always");
-    if (type != LUA_TNONE) {
-      if (type == LUA_TNIL ||
-          lua_toboolean(L, -1)) {
-        dlg->autoNewRow = true;
-      }
-      lua_pop(L, 1);
-    }
+    if (lua_is_key_true(L, 2, "always"))
+      dlg->autoNewRow = true;
+    lua_pop(L, 1);
   }
 
   lua_pushvalue(L, 1);

src/app/script/luacpp.cpp

@@ -93,5 +93,15 @@ void create_mt_getters_setters(lua_State* L,
   ASSERT(lua_gettop(L) == top);
 }
 
+bool lua_is_key_true(lua_State* L, int tableIndex, const char* keyName)
+{
+  bool result = false;
+  int type = lua_getfield(L, tableIndex, keyName);
+  if (type != LUA_TNIL && lua_toboolean(L, -1))
+    result = true;
+  lua_pop(L, 1);
+  return result;
+}
+
 } // namespace script
 } // namespace app

src/app/script/luacpp.h

@@ -128,6 +128,8 @@ void create_mt_getters_setters(lua_State* L,
                                const char* tname,
                                const Property* properties);
 
+bool lua_is_key_true(lua_State* L, int tableIndex, const char* keyName);
+
 #define REG_CLASS_PROPERTIES(L, T) {                                \
     luaL_getmetatable(L, get_mtname<T>());                          \
     create_mt_getters_setters(L, get_mtname<T>(), T##_properties);  \