The slightly modified tables reduce the number of 16-bit values that
convert to an off-by-one 8-bit value.  The "makesRGB.c" code that was used
to generate the tables is now in a contrib/sRGBtables sub-directory.

instead of ANNOUNCE

adding alpha and reducing to 8-bit

on a variety of images, limited documentation (in the header file.)

and pnglibconf.mak

to handle the new PNG_JOIN macro.

the sBIT fields in the test pixel as 0, which resulted in a floating
point division by zero which was irrelevant but causes systems where
FP exceptions cause a crash. Added code to pngvalid to turn on FP
exceptions if the appropriate glibc support is there to ensure this is
tested in the future.

casts. The compression_type parameter is always assigned to, so must
be non-NULL. The cast of the profile length potentially truncated the
value unnecessarily on a 16-bit int system, so the cast of the (byte)
compression type to (int) is specified by ANSI-C anyway.

changes the 'Paeth' reconstruction function to improve the GCC code
generation on x86. The changes are only part of the suggested ones;
just the changes that definitely improve speed and remain simple.
The changes also slightly increase the clarity of the code. In a

adjacent @ signs with a space; this changes the concatentation
from @@@ to PNG_JOIN; that should work with all compiler
preprocessors.

in pngmem.c; pngvalid would attempt to call png_error() if the allocation
of a png_struct or png_info failed. This would probably have led to a
crash.  The pngmem.c implementation of png_malloc() included a cast
to png_size_t which would fail on large allocations on 16-bit systems.

The logical shift fix for Microsoft Visual C is required by other compilers,
so this enables that fix for all compilers when using compile-time constants.
Under MSYS 'byte' is a name declared in a system header file, so we
changed the name of a local variable to avoid the warnings that result.

non-taken code branches and complain about out-of-range shifts.