Support 15bpp color sixel. (-F option)
Replace indent tabs to spaces.
Remove a trailing space.
Add sixel_dither_unref() before return (-1).
Separate sixel_encode_impl() into sixel_encode_header(), sixel_encode_body() and
sixel_encode_footer().
Remove unnecessary check. (nwrite <= 0)
Merge branch 'arakiken' into arakiken-separate-sixel-encode-impl
Merge remote-tracking branch 'origin/develop' into arakiken

Conflicts:
	src/tosixel.c
Remove a performance hack which inserts '\n' after palette definition.
Remove an extra space.
Allocate 'rgbhit' and 'rgb2pal' on heap instead of stack.
Remove unnecessary variables 'src' and 'orig_src'.
Merge pull request #14 from saitoha/arakiken

15bpp color sixel output