in png_handle_sPLT().

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.

and mentioned use of 2-space indentation in macro definitions in
the libpng style guide.

to eliminate bogus complaint about filter_type.

in png_set_gAMA_and_cHRM() where we supply the cHRM constants and
therefore we know they are OK.

also png_check_cHRM_fixed() which was apparently overlooked when it
was created.

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".

about fixing the "#if def " bug

("#if def" should be "#ifdef")

to give Dennis Gustafsson credit for noticing the incorrect typecast
of "filler".