Restore 0-argument love.graphics.newFont/love.font.newRasterizer

Fix subtle bug in luax_convobj in the process, it couldn't deal with 0 arguments
before.

src/common/runtime.cpp

@@ -489,7 +489,8 @@ int luax_convobj(lua_State *L, int idxs[], int n, const char *mod, const char *f
 	lua_call(L, n, 2); // Call the function, n args, one return value (plus optional errstring.)
 	luax_assert_nilerror(L, -2); // Make sure the function returned something.
 	lua_pop(L, 1); // Pop the second return value now that we don't need it.
-	lua_replace(L, idxs[0]); // Replace the initial argument with the new object.
+	if (n > 0)
+		lua_replace(L, idxs[0]); // Replace the initial argument with the new object.
 	return 0;
 }
 

src/modules/font/wrap_Font.cpp

@@ -37,7 +37,7 @@ namespace font
 
 int w_newRasterizer(lua_State *L)
 {
-	if (lua_type(L, 1) == LUA_TNUMBER || lua_type(L, 2) == LUA_TNUMBER)
+	if (lua_type(L, 1) == LUA_TNUMBER || lua_type(L, 2) == LUA_TNUMBER || lua_isnone(L, 1))
 	{
 		// First or second argument is a number: call newTrueTypeRasterizer.
 		return w_newTrueTypeRasterizer(L);
@@ -69,10 +69,10 @@ int w_newTrueTypeRasterizer(lua_State *L)
 	Rasterizer *t = nullptr;
 	TrueTypeRasterizer::Hinting hinting = TrueTypeRasterizer::HINTING_NORMAL;
 
-	if (lua_type(L, 1) == LUA_TNUMBER)
+	if (lua_type(L, 1) == LUA_TNUMBER || lua_isnone(L, 1))
 	{
 		// First argument is a number: use the default TrueType font.
-		int size = luaL_checkint(L, 1);
+		int size = luaL_optint(L, 1, 12);
 
 		const char *hintstr = lua_isnoneornil(L, 2) ? nullptr : luaL_checkstring(L, 2);
 		if (hintstr && !TrueTypeRasterizer::getConstant(hintstr, hinting))