Log Message:
Update libsixel to 1.4.1.

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                   What's new in libsixel-1.4 ?
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* core: SIXEL encoder supports various pixel formats(thanks to @uobikiemukot).

* core: Supports high-color(15bpp output) mode(thanks to @arakiken).

* core: More high compressed output mode is supported(thanks to @arakiken).

* core: Fix typoed API (thanks to @waywardmonkeys).

* core: Fix a minor performance bug (thanks to @arakiken).

* img2sixel: New option -D(--pipe-mode) is introduced.
             with this option, img2sixel continuously read source images from
             stdin.

* img2sixel: Introduce -I(--high-color) option, high-color(15bpp) output mode
             is added(thanks to @arakiken).

* img2sixel: New option -E(--encode-policy) is added. high-compressed
             mode(-E size) is introduced(thanks to @arakiken).

* img2sixel: New option -b(--builtin-palette) is added. built-in support of
             VT-340 palette is added(thanks to @vrtsds).

* img2sixel: Performance inprovement for loading 8bit indexed PNG

* A lots of bug fixes and minor improvements, now tests achieve 90% coverage.