[libpng17]Rearranged png_struct, remove unused members, change png_set_filter

handling png_struct members rearranged - partly to reorder to avoid packing,
partly to put frequently accessed members at the start and partly to make
the grouping more clear. png_set_filter code has been rewritten and the
code shared with png_write_start_row moved to a common function.  Comments
in png.h have been made more clear. Minor fixes to
contrib/libtests/timepng.c and some of the png_*_tRNS logic, including
more error detection in png_set_tRNS.

pngtest.c

@@ -1949,4 +1949,4 @@ main(void)
 #endif
 
 /* Generate a compiler error if there is an old png.h in the search path. */
-typedef png_libpng_version_1_7_0alpha01 Your_png_h_is_not_version_1_7_0alpha01;
+typedef png_libpng_version_1_6_0beta32 Your_png_h_is_not_version_1_6_0beta32;