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
Merge branch 'develop' into stbi-stub
Minor fix
Minor fix
Merge branch 'feature-load-ppm' into sandbox
Merge branch 'stbi-stub' into sandbox

Conflicts:
	converters/loader.c
Minor fix
Load pnm more efficiently
Fix an error caused by duplicated free
Merge branch 'fix-doublefree-error' into stbi-stub
Merge branch 'stbi-stub' into develop