draw: use type aliases for go1.8.typealias builds too

go1.8.typealias is the build tag defined by the unreleased
"Go 1.8 + type aliases" variant in the Go repo's dev.typealias branch.

Change-Id: Iff2566249ecc9a6788b78199743d2005708f1cd7
Reviewed-on: https://go-review.googlesource.com/37511
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/draw/go1_8.go b/draw/go1_8.go
index 95db46f..ec192b7 100644
--- a/draw/go1_8.go
+++ b/draw/go1_8.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !go1.9
+// +build !go1.9,!go1.8.typealias
 
 package draw
 
diff --git a/draw/go1_9.go b/draw/go1_9.go
index 1e878e1..fc548e9 100644
--- a/draw/go1_9.go
+++ b/draw/go1_9.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build go1.9
+// +build go1.9 go1.8.typealias
 
 package draw