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
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.

pngread.c

@@ -1,7 +1,7 @@
 
 /* pngread.c - read a PNG file
  *
- * Copyright (c) 2018 Cosmin Truta
+ * Copyright (c) 2018-2019 Cosmin Truta
  * Copyright (c) 1998-2002,2004,2006-2018 Glenn Randers-Pehrson
  * Copyright (c) 1996-1997 Andreas Dilger
  * Copyright (c) 1995-1996 Guy Eric Schalnat, Group 42, Inc.
@@ -994,6 +994,12 @@ png_read_destroy(png_structrp png_ptr)
    png_ptr->chunk_list = NULL;
 #endif
 
+#if defined(PNG_READ_EXPAND_SUPPORTED) && \
+    defined(PNG_ARM_NEON_IMPLEMENTATION)
+   png_free(png_ptr, png_ptr->riffled_palette);
+   png_ptr->riffled_palette = NULL;
+#endif
+
    /* NOTE: the 'setjmp' buffer may still be allocated and the memory and error
     * callbacks are still set at this point.  They are required to complete the
     * destruction of the png_struct itself.