When PNG transformations are set up, tRNS chunk is converted to alpha channel
early in the process. Later the decision whether to strip alpha channel or not
is made upon checking (color_type & PNG_COLOR_MASK_ALPHA). Thus alpha channel
created by tRNS chunk conversion does not get stripped.
Before this patch check_if_png() comment was kind of misleading because
it talked about the behaviour of png_sig_cmp() which isn't the same as
check_if_png().
