restorable: Do not dispose objects when restoring on browsers

This fix suppresses warnings on the Chrome console.

internal/restorable/images.go

@@ -183,7 +183,9 @@ func (i *images) restore() error {
 
 	// Dispose all the shaders ahead of restoring. A current shader ID and a new shader ID can be duplicated.
 	for s := range i.shaders {
-		s.shader.Dispose()
+		if needsDisposingWhenRestoring {
+			s.shader.Dispose()
+		}
 		s.shader = nil
 	}
 	for s := range i.shaders {
@@ -193,7 +195,9 @@ func (i *images) restore() error {
 	// Dispose all the images ahead of restoring. A current texture ID and a new texture ID can be duplicated.
 	// TODO: Write a test to confirm that ID duplication never happens.
 	for i := range i.images {
-		i.image.Dispose()
+		if needsDisposingWhenRestoring {
+			i.image.Dispose()
+		}
 		i.image = nil
 	}
 

internal/restorable/js.go

@@ -0,0 +1,22 @@
+// Copyright 2020 The Ebiten Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build js
+
+package restorable
+
+// On browsers, disposing resources is not required since the objects are already managed by JavaScript GC and they
+// are already invalidated. Rather, disposing them when restoring causes warnings on the console.
+
+const needsDisposingWhenRestoring = false

internal/restorable/notjs.go

@@ -0,0 +1,19 @@
+// Copyright 2020 The Ebiten Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build !js
+
+package restorable
+
+const needsDisposingWhenRestoring = true