changed Texture_s_load_png back to libpng; and removed cairo as a dependency
Some kinda update

Removed vendor code
collapsed Texture into 1 file (again)
rearranged directory structure
Added StarRuby::Texture#subsample function
Removed the mkrf dependency, and moved ext/starruby_ext to ext/

Also stratos has been removed, since the changes have been added to
StarRuby.
stratos will be rebooted later.