image/png: interlacing support for png.

Fixes #6293.

Image "testdata/benchRGB-interlace.png" was generated by opening "testdata/benchRGB.png" in the editor Gimp and saving it with interlacing enabled.

Benchmark:
BenchmarkDecodeRGB        	     500	   7014194 ns/op	  37.37 MB/s
ok  	pkg/image/png	4.657s

BenchmarkDecodeInterlacing	     100	  10623241 ns/op	  24.68 MB/s
ok  	pkg/image/png	1.339s

LGTM=nigeltao
R=nigeltao, andybons, matrixik
CC=golang-codereviews
https://golang.org/cl/102130044

src/pkg/image/png/reader_test.go

@@ -30,6 +30,7 @@ var filenames = []string{
 	"basn3p01",
 	"basn3p02",
 	"basn3p04",
+	"basn3p04-31i",
 	"basn3p08",
 	"basn3p08-trns",
 	"basn4a08",
@@ -186,6 +187,13 @@ func sng(w io.WriteCloser, filename string, png image.Image) {
 					c = 0
 				}
 			}
+			if c != 0 {
+				for c != 8/bitdepth {
+					b = b << uint(bitdepth)
+					c++
+				}
+				fmt.Fprintf(w, "%02x", b)
+			}
 		}
 		io.WriteString(w, "\n")
 	}
@@ -348,3 +356,7 @@ func BenchmarkDecodePaletted(b *testing.B) {
 func BenchmarkDecodeRGB(b *testing.B) {
 	benchmarkDecode(b, "testdata/benchRGB.png", 4)
 }
+
+func BenchmarkDecodeInterlacing(b *testing.B) {
+	benchmarkDecode(b, "testdata/benchRGB-interlace.png", 4)
+}

src/pkg/image/png/testdata/pngsuite/basn3p04-31i.sng

+#SNG: from basn3p04-31i.png
+IHDR {
+    width: 31; height: 31; bitdepth: 4;
+    using color palette;
+}
+gAMA {1.0000}
+PLTE {
+    ( 34,  0,255)     # rgb = (0x22,0x00,0xff)
+    (  0,255,255)     # rgb = (0x00,0xff,0xff)
+    (136,  0,255)     # rgb = (0x88,0x00,0xff)
+    ( 34,255,  0)     # rgb = (0x22,0xff,0x00)
+    (  0,153,255)     # rgb = (0x00,0x99,0xff)
+    (255,102,  0)     # rgb = (0xff,0x66,0x00)
+    (221,  0,255)     # rgb = (0xdd,0x00,0xff)
+    (119,255,  0)     # rgb = (0x77,0xff,0x00)
+    (255,  0,  0)     # rgb = (0xff,0x00,0x00)
+    (  0,255,153)     # rgb = (0x00,0xff,0x99)
+    (221,255,  0)     # rgb = (0xdd,0xff,0x00)
+    (255,  0,187)     # rgb = (0xff,0x00,0xbb)
+    (255,187,  0)     # rgb = (0xff,0xbb,0x00)
+    (  0, 68,255)     # rgb = (0x00,0x44,0xff)
+    (  0,255, 68)     # rgb = (0x00,0xff,0x44)
+}
+IMAGE {
+    pixels hex
+88885555ccccaaaa77773333eeee9990
+88885555ccccaaaa77773333eeee9990
+88885555ccccaaaa77773333eeee9990
+88885555ccccaaaa77773333eeee9990
+5555ccccaaaa77773333eeee99991110
+5555ccccaaaa77773333eeee99991110
+5555ccccaaaa77773333eeee99991110
+5555ccccaaaa77773333eeee99991110
+ccccaaaa77773333eeee999911114440
+ccccaaaa77773333eeee999911114440
+ccccaaaa77773333eeee999911114440
+ccccaaaa77773333eeee999911114440
+aaaa77773333eeee999911114444ddd0
+aaaa77773333eeee999911114444ddd0
+aaaa77773333eeee999911114444ddd0
+aaaa77773333eeee999911114444ddd0
+77773333eeee999911114444dddd0000
+77773333eeee999911114444dddd0000
+77773333eeee999911114444dddd0000
+77773333eeee999911114444dddd0000
+3333eeee999911114444dddd00002220
+3333eeee999911114444dddd00002220
+3333eeee999911114444dddd00002220
+3333eeee999911114444dddd00002220
+eeee999911114444dddd000022226660
+eeee999911114444dddd000022226660
+eeee999911114444dddd000022226660
+eeee999911114444dddd000022226660
+999911114444dddd000022226666bbb0
+999911114444dddd000022226666bbb0
+999911114444dddd000022226666bbb0
+}