image/png: make the CompressionLevel constants negative, reserving

positive numbers to mean a numeric zlib compression level.

LGTM=bradfitz, ruiu
R=bradfitz, ruiu
CC=golang-codereviews, jeff.allen
https://golang.org/cl/138860043

diff --git a/src/pkg/image/png/writer.go b/src/pkg/image/png/writer.go
index b1a70c79d3f83b54b465a449697c8d328ba168c1..df23270ee975cdcb8b5b38b986a902a02a8b2f40 100644 (file)
--- a/src/pkg/image/png/writer.go
+++ b/src/pkg/image/png/writer.go
@@ -33,10 +33,13 @@ type encoder struct {
 type CompressionLevel int
 
 const (
-       DefaultCompression CompressionLevel = iota
-       NoCompression
-       BestSpeed
-       BestCompression
+       DefaultCompression CompressionLevel = 0
+       NoCompression      CompressionLevel = -1
+       BestSpeed          CompressionLevel = -2
+       BestCompression    CompressionLevel = -3
+
+       // Positive CompressionLevel values are reserved to mean a numeric zlib
+       // compression level, although that is not implemented yet.
 )
 
 // Big-endian.