[libpng16] Fix confusion between "adopted" and "adapted" in new comments.

Glenn thought the "adopted" were typos and changed them all to "adapted".
This reverts the spelling and adds parenthetical comments when each
word first appears in png.h.

pngstruct.h

@@ -147,11 +147,11 @@ typedef const png_colorspace * PNG_RESTRICT png_const_colorspacerp;
    /* The profile description and copyright must be present in all valid ICC
     * profiles, however libpng does not use them so absence is just reported as
     * a warning.  The media white point should be present too, but if it isn't
-    * all we lose is the ability to know if it differs from the adapted white
+    * all we lose is the ability to know if it differs from the adopted white
     * (i.e. the information that the device maxima are actually colored;
     * a non-white substrate for a printer, or an uncorrected scan for example.)
-    * The chromaticAdaptationTag tells us that the adapted white of the original
-    * differs from the PCS adapted white (which is identical to the PCS
+    * The chromaticAdaptationTag tells us that the adopted white of the original
+    * differs from the PCS adopted white (which is identical to the PCS
     * illuminant and should always be D50).
     */
 #  define PNG_ICC_profileDescriptionTag  0x00000010U /* required */