Log Message:
Update to 1.7.0

* Bump API/ABI version

Changelog:
                  ------------------------------
                   What's new in libsixel-1.7 ?
                  ------------------------------

* core: Upgrade stb_image to 2.12.

* core: Upgrade stb_image_write.h to v1.02.

* core: introduce canonical,fast decoder to prevent segmentation faults

* python: add row level API functions to python interface

* img2sixel: fix broken -S option

* img2sixel: fix broken animated GIF resizing feature

* example: add example/drawing

* example: add example/python

* converters: Makefile tweak from yacp(thanks to @fd00)
  \ 
https://github.com/fd00/yacp/blob/896e9bd819b373e8e99fefcec12bd85c73e7f123/libsixel/libsixel-1.6.0-1bl1.src.patch

                  ------------------------------
                   What's new in libsixel-1.6 ?
                  ------------------------------
* img2sixel: Add -R(--gri-limit) option.

* configure: Fix check for gdImageCreateFromPngPtr availability(thanks to @hattya).

* core: Upgrade stb_image to 2.10.

* core: Upgrade stb_image_write.h to v1.01.

* libsixel-config: Add --datarootdir option to libsixel-config.

* img2sixel: Solve encoder option confliction between -m/-e/-I/-b.

* core: Introduce new API for output object, sixel_output_set_gri_arg_limit().

* core: Add new API: sixel_encoder_encode_bytes().

                  ------------------------------
                   What's new in libsixel-1.5 ?
                  ------------------------------
* img2sixel: Add -k(--insecure) option.

* img2sixel: Add -B(--bgcolor) option.

* core: Introduce new API for dither object, sixel_dither_set_transparent().

* core: Respect transparency of paletted images(GIF/PNG).

* core: Introduce API set for generic image maniplation:
    sixel_frame_get_pixels()
    sixel_frame_ref()
    sixel_frame_unref()
    sixel_frame_init()
    sixel_frame_get_pixels()
    sixel_frame_get_palette()
    sixel_frame_get_width()
    sixel_frame_get_height()
    sixel_frame_get_ncolors()
    sixel_frame_get_pixelformat()
    sixel_frame_get_transparent()
    sixel_frame_get_multiframe()
    sixel_frame_get_delay()
    sixel_frame_get_frame_no()
    sixel_frame_get_loop_no()
    sixel_frame_strip_alpha()
    sixel_frame_resize()
    sixel_frame_clip()

* core: Introduce API set for load/write well-known image formats:
    sixel_helper_load_image_file()
    sixel_helper_write_image_file()

* core: Add more high level APIs (File-to-File conversion API):
    sixel_encoder_create()
    sixel_encoder_ref()
    sixel_encoder_unref()
    sixel_encoder_set_cancel_flag()
    sixel_encoder_setopt()
    sixel_encoder_encode()
    sixel_decoder_create()
    sixel_decoder_ref()
    sixel_decoder_unref()
    sixel_decoder_setopt()
    sixel_decoder_decode()

* core: Introduce new APIs for semantic error handling:
    sixel_helper_set_additional_message()
    sixel_helper_get_additional_message()
    sixel_helper_format_error()

* core: Introduce custom allocator APIs:
    sixel_allocator_new()
    sixel_allocator_ref()
    sixel_allocator_unref()
    sixel_allocator_malloc()
    sixel_allocator_calloc()
    sixel_allocator_realloc()
    sixel_allocator_free()

* core: Add new constructor methods that returns SIXELSTATUS for semantic error \ 
handling:
    sixel_output_new()
    sixel_dither_new()
    sixel_frame_new()
    sixel_encoder_new()
    sixel_decoder_new()
    sixel_allocator_new()
    sixel_output_new()
    sixel_dither_new()
    sixel_frame_new()
    sixel_encoder_new()
    sixel_decoder_new()

* core: Improve processing quality of loading paletted PNG images.

* core: Improve processing quality and performance of loading GIF images.

* core: Update stb_image.h to version 2.06

* core: Update stb_image_write.h to version 0.94

* Add Python interface

* Add Perl interface (alpha)

* Add PHP interface (alpha)

* Add Ruby interface (alpha)

* Add new build helper program: libsixel-config

* A lot of bug fixes and minor improvements.
Thanks to @msmhrt, @ttdoda, and @msmhrt.