Add PNM loader imported from kmiya's sixel
Enable pnm format loader in loader.c
Minor fix
Build with frompnm.c
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