* img2sixel.c: Use gdk-pixbuf and libcurl.
* build-gdkpixbuf.sh: Added.
this commit is imported from arakiken/libsixel.
https://bitbucket.org/arakiken/libsixel/commits/fda0bcab1ec2598bada3d102d8f0cf35d848f6ff
Merge branch 'arakiken-sandbox' into arakiken

Conflicts:
	converters/img2sixel.c
Merge branch 'develop' into arakiken
Fix build
Merge branch 'fix-if-pkgconfig-is-unavilable' into arakiken
Merge branch 'issue-2' into arakiken
Merge branch 'sandbox' into kmiya
Merge branch 'feature-load-ppm' into kmiya
Merge branch 'develop' into release-0.19
Bump version
Rebuild
Merge branch 'configure' into release-0.19
Merge branch 'develop' into release-0.19
Merge branch 'develop' into release-0.19
Merge branch 'release-0.19'
Bump version
Merge branch 'develop' into release-0.19
Rebuild configure script
Merge branch 'release-0.19'
Fix a segmentation fault issue reported by @arakiken