Merge branch 'libpng-1.6.28' of https://github.com/catalinr/wxWidgets

This brings in several bug fixes and optimizations (notably the use of SSE),
see src/png/CHANGES for full details.

Closes https://github.com/wxWidgets/wxWidgets/pull/384

docs/changes.txt

@@ -88,6 +88,7 @@ All (GUI):
 - Add support for wxAuiManager and wxAuiPaneInfo to XRC (Andrea Zanellato).
 - Add support for wxSL_MIN_MAX_LABELS and wxSL_VALUE_LABEL to XRC (ousnius).
 - Update Scintilla to v3.6.6 (Paul Kulchenko).
+- Update bundled libpng to 1.6.28 (Catalin Raceanu).
 - Fix vertical scrollbar visibility in wxStyledTextCtrl (yenwu, NewPagodi).
 - Fix bug with not being able to select AUI tab after dragging.
 - Make wxDataViewCtrl::Expand() expand ancestors in native ports too.

docs/contributing/how-to-update-third-party-library.md

@@ -125,3 +125,23 @@ versions there.
 Notice that config.h generated by libpng configure is not used, we build it
 without -DHAVE_CONFIG_H as it works just fine without it on any ANSI C system
 (i.e. anywhere by now).
+
+
+Example updating libpng files under MSW:
+    Since configure looked for zlib, the following 2 steps were also needed:
+    a) get zlib source code (www.zlib.net) to a separate location;
+    b) in zlib root dir run: mingw32-make -fwin32/Makefile.gcc
+       (will generate zlib1.dll)
+1) get libpng source code (libpng.sourceforge.net) to a separate location;
+2) in libpng root dir run:
+   configure --with-libpng-prefix=wx_
+   Alternatively, if configure still cannot find zlib, run:
+   export CPPFLAGS="-I zlib-root-dir"
+   export LDFLAGS="-L zlib-root-dir"
+   configure --with-libpng-prefix=wx_
+3) in libpng root dir run:
+   mingw32-make png.lo
+   (will generate pnglibconf.h and pngprefix.h, among others)
+4) edit pnglibconf.h and change the value of PNG_ZLIB_VERNUM to 0.
+5) copy files from libpng root dir and scripts subfolder to src/png, updating old files only (other new files were not needed);
+6) commit the changes.