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.

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.

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

Broke several remaining "if () statement;" and "else statement;" into
two lines.