Useful for e.g. packing rectangular textures into an atlas. Does not do
rotation.
Not necessarily the awesomest packing method, but better than the totally naive
one in stb_truetype (which is primarily what this is meant to replace).
No memory allocations; uses qsort() and assert() from stdlib. Can override
those by defining STBRP_SORT and STBRP_ASSERT.
This library currently uses the Skyline Bottom-Left algorithm.
Please note: better rectangle packers are welcome! Please implement them to the
same API, but with a different init function.
