apa102: remove allocation in Halt

Document where Draw() cause a memory allocation in the case of an image format
other than NRGBA.

devices/apa102/apa102.go

@@ -124,8 +124,16 @@ func (d *Dev) Write(pixels []byte) (int, error) {
 
 // Halt turns off all the lights.
 func (d *Dev) Halt() error {
-	_, err := d.Write(make([]byte, d.numPixels*3))
+	// Zap out the buffer.
-	return err
+	for i := range d.pixels {
+		if i&3 == 0 {
+			// 0xE0 would probably be fine too.
+			d.pixels[i] = 0xE1
+		} else {
+			d.pixels[i] = 0
+		}
+	}
+	return d.s.Tx(d.rawBuf, nil)
 }
 
 //
@@ -293,6 +301,7 @@ func (l *lut) rasterImg(dst []byte, r image.Rectangle, src image.Image, srcR ima
 	} else {
 		// Generic version.
 		for sX := srcR.Min.X; sX < srcR.Max.X; sX++ {
+			// This causes a memory allocation. There's no way around it.
 			r16, g16, b16, _ := src.At(sX, srcR.Min.Y).RGBA()
 			r := l.r[byte(r16>>8)]
 			g := l.g[byte(g16>>8)]

devices/apa102/apa102_test.go

@@ -674,6 +674,14 @@ func BenchmarkWriteColorfulVariation(b *testing.B) {
 	}
 }
 
+func BenchmarkHalt(b *testing.B) {
+	d, _ := New(spitest.NewRecordRaw(ioutil.Discard), 150, 250, 5000)
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		d.Halt()
+	}
+}
+
 //
 
 type configFail struct {