Use dep to vendor things

vendor/github.com/faiface/pixel/compose.go

@@ -0,0 +1,65 @@
+package pixel
+
+import "errors"
+
+// ComposeTarget is a BasicTarget capable of Porter-Duff composition.
+type ComposeTarget interface {
+	BasicTarget
+
+	// SetComposeMethod sets a Porter-Duff composition method to be used.
+	SetComposeMethod(ComposeMethod)
+}
+
+// ComposeMethod is a Porter-Duff composition method.
+type ComposeMethod int
+
+// Here's the list of all available Porter-Duff composition methods. Use ComposeOver for the basic
+// alpha blending.
+const (
+	ComposeOver ComposeMethod = iota
+	ComposeIn
+	ComposeOut
+	ComposeAtop
+	ComposeRover
+	ComposeRin
+	ComposeRout
+	ComposeRatop
+	ComposeXor
+	ComposePlus
+	ComposeCopy
+)
+
+// Compose composes two colors together according to the ComposeMethod. A is the foreground, B is
+// the background.
+func (cm ComposeMethod) Compose(a, b RGBA) RGBA {
+	var fa, fb float64
+
+	switch cm {
+	case ComposeOver:
+		fa, fb = 1, 1-a.A
+	case ComposeIn:
+		fa, fb = b.A, 0
+	case ComposeOut:
+		fa, fb = 1-b.A, 0
+	case ComposeAtop:
+		fa, fb = b.A, 1-a.A
+	case ComposeRover:
+		fa, fb = 1-b.A, 1
+	case ComposeRin:
+		fa, fb = 0, a.A
+	case ComposeRout:
+		fa, fb = 0, 1-a.A
+	case ComposeRatop:
+		fa, fb = 1-b.A, a.A
+	case ComposeXor:
+		fa, fb = 1-b.A, 1-a.A
+	case ComposePlus:
+		fa, fb = 1, 1
+	case ComposeCopy:
+		fa, fb = 1, 0
+	default:
+		panic(errors.New("Compose: invalid ComposeMethod"))
+	}
+
+	return a.Mul(Alpha(fa)).Add(b.Mul(Alpha(fb)))
+}