errors) and cause it not to fail at the comparison step if libpng lacks support
for writing chunks that it reads from the input (currently only implemented
for compressed text chunks).

a consistent style for (sizeof type) and (sizeof (array))

of strcpy() was accidentally re-introduced in libpng16; this change replaces
it with strncpy().

instead of directly assigning string constants to png_textp members.
This avoids compiler warnings when -Wwrite-strings is enabled.

compiler issues slightly different warnings from those issued by the
current vesions of GCC. This eliminates those warnings by
adding/removing casts and small code rewrites.

changes alter how the tricky allocation of the initial png_struct and png_info
structures are handled. png_info is now handled in pretty much the same
way as everything else, except that the allocations handle NULL return
silently.  png_struct is changed in a similar way on allocation and on
deallocation a 'safety' error handler is put in place (which should never
be required).  The error handler itself is changed to permit mismatches
in the application and libpng error buffer size; however, this means a
silent change to the API to return the jmp_buf if the size doesn't match
the size from the libpng compilation; libpng now allocates the memory and
this may fail.  Overall these changes result in slight code size
reductions; however, this is a reduction in code that is always executed
so is particularly valuable.  Overall on a 64-bit system the libpng DLL
decreases in code size by 1733 bytes.  pngerror.o increases in size by
about 465 bytes because of the new functionality.