Default video output format creates artifacts on colored output

[leios]

for example, if you color the ball in the smear example, you get 2 balls:

This has to do with the pixel_format = "yuv420p" encoder option, which is enabled by default. I am not sure what to do about this because it's also necessary for #11, so I need to play with which options are best.

Note that most people will probably choose to output images and then bind them together themselves or else choose different encoding options for final renders, so this is a low priority issue for now.