[libpng16] Replace the remaining uses of png_size_t with size_t

In v1.6.0, size_t became a required type. It should be used
consistently. To maintain backwards compatibility, png_size_t
is still maintained in deprecated form.
diff --git a/pngstruct.h b/pngstruct.h
index b5cabe9..26ea2d5 100644
--- a/pngstruct.h
+++ b/pngstruct.h
@@ -47,7 +47,7 @@
 /* zlib.h declares a magic type 'uInt' that limits the amount of data that zlib
  * can handle at once.  This type need be no larger than 16 bits (so maximum of
  * 65535), this define allows us to discover how big it is, but limited by the
- * maximuum for png_size_t.  The value can be overridden in a library build
+ * maximum for size_t.  The value can be overridden in a library build
  * (pngusr.h, or set it in CPPFLAGS) and it works to set it to a considerably
  * lower value (e.g. 255 works).  A lower value may help memory usage (slightly)
  * and may even improve performance on some systems (and degrade it on others.)
@@ -214,7 +214,7 @@
    png_uint_32 height;        /* height of image in pixels */
    png_uint_32 num_rows;      /* number of rows in current pass */
    png_uint_32 usr_width;     /* width of row at start of write */
-   png_size_t rowbytes;       /* size of row in bytes */
+   size_t rowbytes;           /* size of row in bytes */
    png_uint_32 iwidth;        /* width of current interlaced row in pixels */
    png_uint_32 row_number;    /* current row in interlace pass */
    png_uint_32 chunk_name;    /* PNG_CHUNK() id of current chunk */
@@ -232,7 +232,7 @@
    png_bytep try_row;    /* buffer to save trial row when filtering */
    png_bytep tst_row;    /* buffer to save best trial row when filtering */
 #endif
-   png_size_t info_rowbytes;  /* Added in 1.5.4: cache of updated row bytes */
+   size_t info_rowbytes;      /* Added in 1.5.4: cache of updated row bytes */
 
    png_uint_32 idat_size;     /* current IDAT size for read */
    png_uint_32 crc;           /* current chunk CRC value */
@@ -328,10 +328,10 @@
    png_bytep current_buffer;         /* buffer for recently used data */
    png_uint_32 push_length;          /* size of current input chunk */
    png_uint_32 skip_length;          /* bytes to skip in input data */
-   png_size_t save_buffer_size;      /* amount of data now in save_buffer */
-   png_size_t save_buffer_max;       /* total size of save_buffer */
-   png_size_t buffer_size;           /* total amount of available input data */
-   png_size_t current_buffer_size;   /* amount of data now in current_buffer */
+   size_t save_buffer_size;          /* amount of data now in save_buffer */
+   size_t save_buffer_max;           /* total size of save_buffer */
+   size_t buffer_size;               /* total amount of available input data */
+   size_t current_buffer_size;       /* amount of data now in current_buffer */
    int process_mode;                 /* what push library is currently doing */
    int cur_palette;                  /* current push library palette index */
 
@@ -451,7 +451,7 @@
 #endif
 
 /* New member added in libpng-1.2.26 */
-  png_size_t old_big_row_buf_size;
+   size_t old_big_row_buf_size;
 
 #ifdef PNG_READ_SUPPORTED
 /* New member added in libpng-1.2.30 */