[libpng15] Only warn about image too wide if warning was not already issued.
[libpng15] Revised png_check_IHDR() to use PNG_SIZE_MAX instead of

PNG_UINT_32_MAX in the test for potential overflow in PNG_ROWBYTES.
[libpng15] Removed potentially misleading warning from png_check_IHDR().
[libpng15] Imported from libpng-1.5.18beta04.tar
[libpng15] Bump version to 1.5.18beta05
[libpng15] Updated scripts/makefile.* to use CPPFLAGS (Cosmin)
[libpng15] Imported from libpng-1.5.18beta05.tar