to check info_ptr members instead of the contents of the returned parameters.

to add a test for user passing a 16-bit height varible to png_get_IHDR()
which causes an overflow into the high bytes of the width variable.

to eliminate bogus complaint about filter_type.

Move redundant IHDR checking into new png_check_IHDR() in png.c
and report all errors found in the IHDR data.  Report problems
with width and height separately.

and always use png_calloc().  The macros were only there for testing,
since libpng-1.4.0beta48 and no problems have been reported.

Always use local arrays, which has been the default behavior
for a long time.

They were only useful for obtaining compatibility for old applications
that used libpng-1.0.x versions prior to the implementation of "free_me".

except in pngconf.h where they can be used to request that
PNG_FEATURE_SUPPORTED not be defined.