Update README
Separate sixel_encode_impl() into sixel_encode_header(), sixel_encode_body() and
sixel_encode_footer(). Cherry-picked from 339d958.

Conflicts:
	src/tosixel.c
Replace some indent tabs to spaces
Strip unused variables
Remove unnecessary check. (nwrite <= 0)
Cherry-picked from cf00bed

Conflicts:
	src/tosixel.c
Drop unnecessary variable sixel_output_t::conv_palette
Fix a segmentation fault occurs when using libpng loader
Merge branch 'develop' into feature-amalgamation

Conflicts:
	src/tosixel.c