[libpng17] Updated warning in png_check_IHDR() to say "may be too large"

instead of "is too large".
[libpng17] Only issue a warning about image width if an error has not already

been found in IHDR.
[libpng17] Revised png_check_IHDR() to use PNG_SIZE_MAX instead of

PNG_UINT_32_MAX in the test for potential overflow in PNG_ROWBYTES.
[libpng17] Removed potentially misleading warning from png_check_IHDR().
[libpng17] Imported from libpng-1.7.0beta27.tar
[libpng17] Bump version to 1.7.0beta28
[libpng17] Fix typo in CHANGES and ANNOUNCE
[libpng17] Updated scripts/makefile.* to use CPPFLAGS (Cosmin)
[libpng17] Added clang attribute support (Cosmin).
[libpng17] Correct a comment in pngconf.h
[libpng17] Checked in clang attribute support.
[libpng17] Imported from libpng-1.7.0beta28.tar
[libpng17] Bump version to 1.7.0beta29