[libpng16] arm: Use png_aligncastconst for an upcast in Neon intrin…

…sics

This changes the arm/palette_neon_intrinsics.c code from using a bare
cast to using the portable macro `png_aligncastconst`.  This removes
the `-Wcast-align` warning.

The cast is safe because the pointer was allocated by `malloc(1024)`,
and `malloc` is required to return a pointer with sufficient alignment
for anything the allocation can accomodate.

Nevertheless, this is a quick and dirty fix; the use of `png_bytep` in
the `png_struct` declaration of the pointer is machine-specific.  Other
architectures may need different types and, consequently, the only type
which is portable to other companies' CPUs is `(void*)`.

This is a cherry-pick of commit a604b12
from branch 'libpng18'.

Reviewed-by: Cosmin Truta <ctruta@gmail.com>
Signed-off-by: John Bowler <jbowler@acm.org>
Signed-off-by: Cosmin Truta <ctruta@gmail.com>

arm/palette_neon_intrinsics.c

@@ -64,7 +64,7 @@ png_do_expand_palette_rgba8_neon(png_structrp png_ptr, png_row_infop row_info,
 {
    png_uint_32 row_width = row_info->width;
    const png_uint_32 *riffled_palette =
-      (const png_uint_32 *)png_ptr->riffled_palette;
+      png_aligncastconst(png_const_uint_32p, png_ptr->riffled_palette);
    const png_uint_32 pixels_per_chunk = 4;
    png_uint_32 i;