stb_image_ffi 1.0.1

Very naive bindings for `stb_image.h`. Implements only `stbi_set_flip_vertically_on_load` and `stb_load` since these are the 2 functions I need to load an image as a texture in OpenGL. The reason I made this instead of using [stb-image](https://rubygems.org/gems/stb-image) is because stb-image didn't implement `stbi_set_flip_vertically_on_load`, as far as I can tell, which is useful/necessary when wanting to load image data in a format immediately consumable by OpenGL (see Example in README).