- Fixed rgb_to_gray checks and added tRNS checks to pngvalid.c.
+ libpng-rgb_to_gray-checks.patch

- updated to 1.6.27:
Corrected the width limit calculation in png_check_IHDR().
Removed user limits from pngfix. Also pass NULL pointers to
png_read_row to skip the unnecessary row de-interlace stuff.
Implement previously untested cases of libpng transforms in pngvalid.c
Fixed byte order in 2-byte filler, in png_do_read_filler().
Made the check for out-of-range values in png_set_tRNS() detect
values that are exactly 2^bit_depth, and work on 16-bit platforms.
Merged some parts of libpng-1.6.17beta01 and libpng-1.7.0beta47.
Added #ifndef __COVERITY__ where needed in png.c, pngrutil.c and
pngset.c to avoid warnings about dead code.
Do not build png_product2() when it is unused.
Display user limits in the output from pngtest.
Eliminated the PNG_SAFE_LIMITS macro and restored the 1-million-column
and 1-million-row default limits in pnglibconf.dfa, that can be reset
by the user at build time or run time. This provides a more robust
defense against DOS and as-yet undiscovered overflows.
Added PNG_WRITE_CUSTOMIZE_COMPRESSION_SUPPORTED macro, on by default.
Allow user to call png_get_IHDR() with NULL arguments (Reuben Hawkins).
Moved png_set_filter() prototype into a PNG_WRITE_SUPPORTED block
of png.h.
Free the unknown_chunks structure even when it contains no data.
Fixed simplified 8-bit-linear to sRGB alpha. The calculated alpha
value was wrong. It's not clear if this affected the final stored
value; in the obvious code path the upper and lower 8-bits of the
alpha value were identical and the alpha was truncated to 8-bits
rather than dividing by 257 (John Bowler).