[devel]  Moved "#include png.h" inside pngpriv.h and removed "#include png.h"

from the source files, along with "#define PNG_EXPOSE_INTERNAL_STRUCTURES"
and "#define PNG_NO_PEDANTIC_WARNINGS" (John Bowler).
Also created new pngdebug.h and moved debug definitions there.
diff --git a/pngmem.c b/pngmem.c
index 1b59432..21ec12f 100644
--- a/pngmem.c
+++ b/pngmem.c
@@ -17,12 +17,10 @@
  * identify the replacement functions.
  */
 
-#define PNG_EXPOSE_INTERNAL_STRUCTURES
-#define PNG_NO_PEDANTIC_WARNINGS
-#include "png.h"
-#if defined(PNG_READ_SUPPORTED) || defined(PNG_WRITE_SUPPORTED)
 #include "pngpriv.h"
 
+#if defined(PNG_READ_SUPPORTED) || defined(PNG_WRITE_SUPPORTED)
+
 /* Borland DOS special memory handler */
 #if defined(__TURBOC__) && !defined(_Windows) && !defined(__FLAT__)
 /* If you change this, be sure to change the one in png.h also */