STBI Image issues

[leios]

#20 introduces the STBI library for image io (https://github.com/nothings/stb). This allows for output in png, jpg, bmp, hrd, and tga.

A few issues:

• I did not enable support for tga or hdr because I have never used them personally. Also, hrd requires floats instead of chars, so I need to add a new type of color (maybe quibble_hrd_color or something).
• The png output is actually relatively slow (4x slower than Julia). Solutions here could include writing directly to an FFMPEG stream or using a faster image io lib