[libpng16] Bump version to 1.6.0beta20

ANNOUNCE

@@ -1,5 +1,5 @@
 
-Libpng 1.6.0beta19 - March 19, 2012
+Libpng 1.6.0beta20 - March 19, 2012
 
 This is not intended to be a public release.  It will be replaced
 within a few weeks by a public version or by another test version.
@@ -9,20 +9,20 @@ Files available for download:
 Source files with LF line endings (for Unix/Linux) and with a
 "configure" script
 
-   1.6.0beta19.tar.xz (LZMA-compressed, recommended)
-   1.6.0beta19.tar.gz
-   1.6.0beta19.tar.bz2
+   1.6.0beta20.tar.xz (LZMA-compressed, recommended)
+   1.6.0beta20.tar.gz
+   1.6.0beta20.tar.bz2
 
 Source files with CRLF line endings (for Windows), without the
 "configure" script
 
-   lp160b19.7z  (LZMA-compressed, recommended)
-   lp160b19.zip
+   lp160b20.7z  (LZMA-compressed, recommended)
+   lp160b20.zip
 
 Other information:
 
-   1.6.0beta19-README.txt
-   1.6.0beta19-LICENSE.txt
+   1.6.0beta20-README.txt
+   1.6.0beta20-LICENSE.txt
 
 Changes since the last public release (1.5.7):
 
@@ -318,7 +318,7 @@ Version 1.6.0beta18 [March 16, 2012]
     read benign errors to warnings (regardless of the system default, unless
     this is disabled in which case the simplified API can't be built.)
 
-Version 1.6.0beta19 [March 19, 2012]
+Version 1.6.0beta19 [March 18,2012]
   Work around for duplicate row start calls; added warning messages.
     This turns on PNG_FLAG_DETECT_UNINITIALIZED to detect app code that
     fails to call one of the 'start' routines (not enabled in libpng-1.5
@@ -339,6 +339,8 @@ Version 1.6.0beta19 [March 19, 2012]
   Avoid the double gamma correction warning in the simplified API.
     This allows the --strict option to pass in the pngstest checks
 
+Version 1.6.0beta20 [March 19, 2012]
+
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