[libpng17] Made 8-bit compose and rgb_to_grayscale accuracy improvements.

These changes cause 16-bit arithmetic to be used for 8-bit data in the gamma
    corrected compose and grayscale operations.  The arithmetic errors have
    three sources all of which are fixed in this commit:
    1) 8-bit linear calculations produce massive errors for lower intensity
       values.
    2) The old 16-bit "16 to 8" gamma table code erroneously wrote the lowest
       output value into a table entry which corresponded to multiple output
       values (so where the value written should have been the closest to the
       transformed input value.)
    3) In a number of cases the code to access the 16-bit table did not round;
       it did a simple shift, which was wrong and made the side effects of (2)
       even worse.
  The new gamma code does not have the 16-to-8 problem at the cost of slighly
    more calculations and the algorithm used to minimize the number of
    calculations has been extended to all the 16-bit tables; it has advantages
    for any significant gamma correction.

pngwio.c

@@ -151,6 +151,7 @@ png_set_write_fn(png_structrp png_ptr, png_voidp io_ptr,
 #  endif
 #endif /* PNG_WRITE_FLUSH_SUPPORTED */
 
+#ifdef PNG_READ_SUPPORTED
    /* It is an error to read while writing a png file */
    if (png_ptr->read_data_fn != NULL)
    {
@@ -160,5 +161,6 @@ png_set_write_fn(png_structrp png_ptr, png_voidp io_ptr,
           "Can't set both read_data_fn and write_data_fn in the"
           " same structure");
    }
+#endif
 }
 #endif /* PNG_WRITE_SUPPORTED */