ebitenutil: Add import image/png for backward compatibility

Fixes #518

ebitenutil/import.go

@@ -0,0 +1,21 @@
+// Copyright 2018 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.
+
+package ebitenutil
+
+import (
+	// Import image/png for backward compatibility (#500, #518)
+	// TODO: Deprecate and remove this at the next major version (2.0).
+	_ "image/png"
+)