image: use built-in clear to simplify code

src/image/draw/draw.go

@@ -1078,9 +1078,7 @@ func drawPaletted(dst Image, r image.Rectangle, src image.Image, sp image.Point,
 		// Recycle the quantization error buffers.
 		if floydSteinberg {
 			quantErrorCurr, quantErrorNext = quantErrorNext, quantErrorCurr
-			for i := range quantErrorNext {
-				quantErrorNext[i] = [4]int32{}
-			}
+			clear(quantErrorNext)
 		}
 	}
 }

src/image/gif/reader.go

@@ -406,10 +406,8 @@ func (d *decoder) readImageDescriptor(keepAllFrames bool) error {
 			// seem OK with this, so we enlarge the palette with
 			// transparent colors. See golang.org/issue/15059.
 			p := make(color.Palette, ti+1)
-			copy(p, m.Palette)
-			for i := len(m.Palette); i < len(p); i++ {
-				p[i] = color.RGBA{}
-			}
+			i := copy(p, m.Palette)
+			clear(p[i:])
 			m.Palette = p
 		}
 	}

src/image/gif/writer.go

@@ -220,10 +220,7 @@ func encodeColorTable(dst []byte, p color.Palette, size int) (int, error) {
 	n := log2Lookup[size]
 	if n > len(p) {
 		// Pad with black.
-		fill := dst[3*len(p) : 3*n]
-		for i := range fill {
-			fill[i] = 0
-		}
+		clear(dst[3*len(p) : 3*n])
 	}
 	return 3 * n, nil
 }

src/image/jpeg/huffman.go

@@ -131,9 +131,7 @@ func (d *decoder) processDHT(n int) error {
 		}
 
 		// Derive the look-up table.
-		for i := range h.lut {
-			h.lut[i] = 0
-		}
+		clear(h.lut[:])
 		var x, code uint32
 		for i := uint32(0); i < lutSize; i++ {
 			code <<= 1

src/image/png/writer.go

@@ -295,12 +295,6 @@ func filter(cr *[nFilter][]byte, pr []byte, bpp int) int {
 	return filter
 }
 
-func zeroMemory(v []uint8) {
-	for i := range v {
-		v[i] = 0
-	}
-}
-
 func (e *encoder) writeImage(w io.Writer, m image.Image, cb int, level int) error {
 	if e.zw == nil || e.zwLevel != level {
 		zw, err := zlib.NewWriterLevel(w, level)
@@ -359,7 +353,7 @@ func (e *encoder) writeImage(w io.Writer, m image.Image, cb int, level int) erro
 		e.pr = make([]uint8, sz)
 	} else {
 		e.pr = e.pr[:sz]
-		zeroMemory(e.pr)
+		clear(e.pr)
 	}
 	pr := e.pr