Remove unused PictureCommand enum

[jsuarezruiz]

It is a public enum that is not used and can cause confusion. @jonlipsky What do you think here?, what was the purpose of the enum?

Fixes #129

[jonlipsky]

@jsuarezruiz It's not currently in use, but it will be in use soon.

I wanted the API to stabilize before I brought over the code that will use it. Its intended use is to provide something similar to SkPicture within Skia: a way to serialize and replay a set of drawing commands.

It will be key to addressing these issues:
#244
#245

Additionally, there is one other use that you may find interesting that I use in all of my personal apps. Instead of bundling images in a vector format such as SVG or PDF (on iOS), I bundle my images in my apps using this binary "picture" representation, which is faster to load/parse and is faster to render.

[jsuarezruiz]

Got it. Feel free to close the PR.