[libpng16] Changed png_memcpy to C assignment where appropriate. Changed all

those uses of png_memcpy that were doing a simple assignment to assignments
(all those cases where the thing being copied is a non-array C L-value.)
Added some error checking to png_set_*() routines and removed the
reference to the non-exported function png_memcpy() from example.c. Fixed
the Visual C 64-bit build - it requires jmp_buf to be aligned, but it had
become misaligned.

pngstruct.h

@@ -29,10 +29,10 @@
 struct png_struct_def
 {
 #ifdef PNG_SETJMP_SUPPORTED
+   jmp_buf jmp_buf_local;     /* New name in 1.6.0 for jmp_buf in png_struct */
    png_longjmp_ptr longjmp_fn;/* setjmp non-local goto function. */
    jmp_buf *jmp_buf_ptr;      /* passed to longjmp_fn */
    size_t jmp_buf_size;       /* size of the above, if allocated */
-   jmp_buf jmp_buf_local;     /* New name in 1.6.0 for jmp_buf in png_struct */
 #endif
    png_error_ptr error_fn;    /* function for printing errors and aborting */
 #ifdef PNG_WARNINGS_SUPPORTED
@@ -264,7 +264,7 @@ struct png_struct_def
 #endif
 
 #ifdef PNG_HANDLE_AS_UNKNOWN_SUPPORTED
-   int num_chunk_list;
+   unsigned int num_chunk_list;
    png_bytep chunk_list;
 #endif