Fixed corrupted output of palettized GIFs

* Added palettization options

** Using `avPixFmtRgb8` results in a small file

** Using the default pixel format (`avPixFmtPal8`) results in a good-looking,
   fairly large file thanks to JuicyPixels's `palettize` function.

** Setting the `epPreset` field of the `EncodingParams` value passed to
   `frameWriter` to `"dither"` results in an even prettier, even larger GIF
   file (again, thanks to JuicyPixels's `palettize` function).

** See the `demo/Raster.hs` for examples.