[devel] Changes to remove gcc warnings (John Bowler)

Certain optional gcc warning flags resulted in warnings in libpng code.
With these changes only -Wconversion and -Wcast-qual cannot be turned on.
Changes are trivial rearrangements of code.  -Wconversion is not possible
for pngrutil.c (because of the widespread use of += et al on variables
smaller than (int) or (unsigned int)) and -Wcast-qual is not possible
with pngwio.c and pngwutil.c because the 'write' callback and zlib
compression both fail to declare their input buffers with 'const'.

pngread.c

@@ -618,6 +618,7 @@ png_read_row(png_structp png_ptr, png_bytep row, png_bytep dsp_row)
             }
             break;
 
+         default:
          case 6:
             if (!(png_ptr->row_number & 1))
             {