mobile/ebitenmobileview: Add a dummy file for non-mobile environments

mobile/ebitenmobileview/doc.go

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+// Copyright 2019 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 ebitenmobileview offers functions for OpenGL/Metal view of mobiles.
+//
+// The functions are not intended for public usages.
+// There is no guarantee of backward compatibility.
+package ebitenmobileview