[libpng16] Removed png_get_signature() and info_ptr->signature from WRITE-only

builds.  Restored png_get_channels() to WRITE-only builds.

png.h

@@ -2008,11 +2008,9 @@ PNG_EXPORT(113, void, png_set_rows, (png_const_structrp png_ptr,
     png_inforp info_ptr, png_bytepp row_pointers));
 #endif
 
-#ifdef PNG_READ_SUPPORTED
 /* Returns number of color channels in image. */
 PNG_EXPORT(114, png_byte, png_get_channels, (png_const_structrp png_ptr,
     png_const_inforp info_ptr));
-#endif
 
 #ifdef PNG_EASY_ACCESS_SUPPORTED
 /* Returns image width in pixels. */

pngget.c

@@ -423,7 +423,6 @@ png_get_pHYs_dpi(png_const_structrp png_ptr, png_const_inforp info_ptr,
 #endif  /* PNG_EASY_ACCESS_SUPPORTED */
 
 
-#ifdef PNG_READ_SUPPORTED
 png_byte PNGAPI
 png_get_channels(png_const_structrp png_ptr, png_const_inforp info_ptr)
 {
@@ -433,6 +432,7 @@ png_get_channels(png_const_structrp png_ptr, png_const_inforp info_ptr)
    return (0);
 }
 
+#ifdef PNG_READ_SUPPORTED
 png_const_bytep PNGAPI
 png_get_signature(png_const_structrp png_ptr, png_const_inforp info_ptr)
 {

pnginfo.h

@@ -73,7 +73,10 @@ struct png_info_def
    png_byte channels;       /* number of data channels per pixel (1, 2, 3, 4) */
    png_byte pixel_depth;    /* number of bits per pixel */
    png_byte spare_byte;     /* to align the data, and for future use */
+
+#ifdef PNG_READ_SUPPORTED
    png_byte signature[8];   /* magic bytes read by libpng from start of file */
+#endif
 
    /* The rest of the data is optional.  If you are reading, check the
     * valid field to see if the information in these are valid.  If you