[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/pngwio.c b/pngwio.c
index fd3c5fe..a0be6f8 100644
--- a/pngwio.c
+++ b/pngwio.c
@@ -18,12 +18,10 @@
  * them at run time with png_set_write_fn(...).
  */
 
-#define PNG_EXPOSE_INTERNAL_STRUCTURES
-#define PNG_NO_PEDANTIC_WARNINGS
-#include "png.h"
-#ifdef PNG_WRITE_SUPPORTED
 #include "pngpriv.h"
 
+#ifdef PNG_WRITE_SUPPORTED
+
 /* Write the data to whatever output you are using.  The default routine
  * writes to a file pointer.  Note that this routine sometimes gets called
  * with very small lengths, so you should implement some kind of simple