Introduce new APIs: sixel_output_{get,set}_skip_dcs_envelope
Update libtool minor version
Skip an unknown code(0x3c) at GIF decoder
Merge branch 'hotfix-for-corrupted-gif' into release-1.2
Merge branch 'feature-customize-dcs-envelope' into release-1.2

Conflicts:
	configure
	configure.ac
	include/sixel.h.in
	src/tosixel.c