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 revised libpng style to mention not using the TAB character.

to avoid compiler complaints about doubly declaring things "const".

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

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