Fix a memory leak in the riffled palette optimization on ARM; refactor

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

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.
6 files changed