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.

and removed a redundant assignment (Mans Rullgard).

is not byte aligned, while reading. Prior to libpng-1.5.6 libpng would
overwrite the end of the image if the row width is not an exact multiple
of 8 bits and the image is not interlaced.

These fixes attend to most of the errors revealed in pngvalid, however doing
the gamma work twice results in inaccuracies that can't be easily fixed.
There is now a warning in the code if this is going to happen.

in compressed text writing

It is too risky. Exactly how png_ptr->zbuf is used is under control of the
individual chunk implementation and there could easily be significant changes
within a major release.

introduced in 1.5.6bet01.

small percentage for 16-bit and 32-bit pixels in the typical case where the
output row buffers are appropriately aligned. The optimization was not
previously possible because the png_struct buffer was always misaligned.

caused by a bug in the code that attempted to align it; the code needs to
subtract one from the pointer to take account of the filter byte prepended to
each row.

generality of the code, allowing it to be optimized for Adam7 interlace.  The
masks passed to png_combine_row() are now generated internally, avoiding
some code duplication and localizing the interlace handling somewhat.

not supported.