Merge branch 'upstream-png' into fix-png-optimization-flags

* upstream-png:
  png 2019-05-28 (4d63877d)

ThirdParty/png/vtkpng/README.kitware.md

@@ -9,3 +9,4 @@ process.
   * Ignore whitespace errors for VTK's commit checks.
   * Integrate the CMake build with VTK's module system.
   * Mangle all exported symbols to have a `vtkpng_` prefix.
+  * Avoid processor-specific optimizations (NEON, SSE, etc.)

ThirdParty/png/vtkpng/vtk_png_mangle.h

 #ifndef vtk_png_mangle_h
 #define vtk_png_mangle_h
 
+#define PNG_ARM_NEON_OPT 0
+#define PNG_MIPS_MSA_OPT 0
+#define PNG_POWERPC_VSX_OPT 0
+#define PNG_INTEL_SSE_OPT 0
+
 #define png_access_version_number vtkpng_png_access_version_number
 #define png_app_error vtkpng_png_app_error
 #define png_app_warning vtkpng_png_app_warning