* 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 'travis' into configure-gdkpixbuf
Merge branch 'sandbox' into feature-load-gif-animation
Merge branch 'release-0.19'
Merge branch 'develop'
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 'release-0.19'
Add PNM loader imported from kmiya's sixel
Enable pnm format loader in loader.c
Minor fix
Build with frompnm.c
Don't use stbi_load_from_file
Add functions detect pnm/sixel image format
Do not use stbi loader if image format is pnm/sixel
Minor fix
Update minor version
Merge branch 'issue-2' into release-0.19