The memory leak remains fixed, but the associated refactoring is
being reverted. Moving the initialization of the riffled palette
from png_do_read_transformations to png_init_palette_transformations
has caused a regression in some of the test programs.

Although png_init_palette_transformations is the proper place to
perform this initialization, and the test programs are technically
incorrect to fail, we are still undoing that refactoring for the
time being.

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.

Also apply style and formatting fixes

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

In v1.6.0, compiler support for const became a requirement.
It should be used consistently. To maintain backwards compatibility,
PNG_CONST is still maintained in deprecated form.

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.

found via `codespell-q 3`
please review

Some are user facing. Some are in actual code. Most are in source comments. Also, please double check the changes in contrib/tools/pngfix.c

png_calloc() that were png_uint_32 instead of png_alloc_size_t
(Bug report by "irwir" in Github libpng issue #175).

  result when integers appear on both sides of a compare.  Worked around the
  others by forcing the strict-overflow setting in the relevant functions to
  a level where they are not reported.
Changed "FALL THROUGH" comments to "FALLTHROUGH" because GCC doesn't like
  the space.
Worked around some C-style casts from (void*) because g++ 5.4.0 objects
  to them.
Increased the buffer size for 'sprint' to pass the gcc 7.1.0 'sprint
  overflow' check that is on by default with -Wall -Wextra.