Add euclidean_distance_rgba

This method was recently added to ChunkyPNG[1]. From that commit
message:

  This method simply takes the Euclidean distance between the RGBA
  channels of 2 colors, which gives us a measure of how different the
  two colors are.

  Although it would be more perceptually accurate to calculate a proper
  Delta E in Lab colorspace, this method should serve many use-cases
  while avoiding the overhead of converting RGBA to Lab.

This commit adds the C implementation of this method to OilyPNG.

[1]: https://github.com/wvanbergen/chunky_png/commit/6e350b2f4a56fb3d07cf2960da51c49a389de1dc

Note that the specs won't pass until a new version of ChunkyPNG with
`euclidean_disatance_rgba` is released, and OilyPNG is updated to
depend on this version.