Before Go 1.7 there is an extra offset needed to decode blocks correctly

after the embedded compressed data. In post 1.7 this bug was fixed.
We use build tags here to support both cases.

common/block.go

@@ -137,7 +137,12 @@ func NewDecodedBlock(data []byte, colors *Colors) (db *DecodedBlock, err error)
 		return
 	}
 
-	offset += buf.Pos + 4
+	// There is a bug before Go 1.7 that enforces
+	// to add 4 as an offset after the compressed
+	// geometry data. This is resolved via build tags
+	// and definitions in pre17offset.go and
+	// post17offset.go.
+	offset += buf.Pos + afterCompressOfs
 	buf.Pos = 0
 	if offset >= dataLen {
 		return nil, ErrBlockTruncated

common/post17offset.go

@@ -0,0 +1,10 @@
+// Copyright 2016 by Sascha L. Teichmann
+// Use of this source code is governed by the MIT license
+// that can be found in the LICENSE file.
+
+// +build go1.7
+
+package common
+
+// afterCompressOfs is not necessary after Go 1.7.
+const afterCompressOfs = 0

common/pre17offset.go

@@ -0,0 +1,10 @@
+// Copyright 2016 by Sascha L. Teichmann
+// Use of this source code is governed by the MIT license
+// that can be found in the LICENSE file.
+
+// +build !go1.7
+
+package common
+
+// afterCompressOfs is necessary before Go 1.7.
+const afterCompressOfs = 4