The changes include support for function attributes in VC in addition to
those already present in GCC - necessary because without these some
warnings are unavoidable. Fixes include signed/unsigned fixes in
pngvalid and checks with gcc -Wall -Wextra -Wunused.
VC requires function attributes on function definitions as well as
declarations, PNG_FUNCTION has been added to enable this and the
relevant function definitions changed.
that were recently introduced, mostly resulting from
safe but uncast assignments to shorter integers. Also added a zlib
VStudio release library project because the latest zlib Official Windows
build does not include such a thing.
