Move deallocation of riffled_palette from png_write_destroy to
png_read_destroy. The reader (not the writer) is the owner of
riffled_palette.

Move allocation and initialization of riffled_palette from
png_do_read_transformations to png_init_palette_transformations.

Allow riffled_palette inside png_struct only if the ARM Neon
optimizations are enabled.

Rename png_riffle_palette_rgba to png_riffle_palette_rgba8, etc.,
to better indicate the strict applicability of these routines.

Fix an unused parameter warning in the build configurations where
riffled palette optimization is not enabled.

Fix indentation.

ARM-specific optimization processes 8 or 4 pixels at once.
Improves performance by around 10-22% on a recent ARM Chromebook.

As per the const correctness rules, top-level const-ness of data
in automatic scopes does not propagate outside of these scopes
(unlike const-ness at lower levels, such as pointers to const data).

Previously, const was used liberally, but inconsistently across the
libpng codebase. Using const wherever applicable is not incorrect.
However, _consistent_ use of const is difficult to maintain in such
conditions.

In conclusion, we shall continue to use const only where doing so is
strictly necessary:

1. If a function guarantees that it will not modify an argument
   passed by pointer, the corresponding function parameter should be
   a pointer-to-const (const T *).

2. Static data should not be modified, therefore it should be const.

Reference:
Google C++ Style Guide
https://google.github.io/styleguide/cppguide.html#Use_of_const

This information is maintained by the version control system.

In v1.6.0, size_t became a required type. It should be used
consistently. To maintain backwards compatibility, png_size_t
is still maintained in deprecated form.

Always define PNG_INTEL_SSE_OPT and PNG_INTEL_SSE_IMPLEMENTATION before
comparing them with 0.

function (Suggested by Max Stepin).

PNG_ARM_NEON_IMPLEMENTATION == 2 to exclude some definitions that assembler
cannot recognize.

("typedef" directive is unrecognized by the assembler).

__ppc64__ -> __PPC64__

Configure libpng with "configure --enable-intel-sse" or compile
libpng with "-DPNG_INTEL_SSE" in CPPFLAGS to enable it.  This patch was
previously applied to libpng-1.6.28rc03 but withdrawn to allow time for QA.