attempt to fix: video/canvas option checks

[nodgear]

Pull request following from #12

[ThaUnknown]

I don't like this, a lot, the general idea is that if the user specifies their own canvas they position it themselves, so the parent shouldn't even be created in that case, what you want to do is spawn a canvas and have it as a reference once its appended, which is... useless as you can just simply do

const instance = new JASSUB(opts)
const canvas = instance._canvas

and you want to do

const canvas = document.createElement('canvas')
const instance = new JASSUB({ ...opts, canvas })

I don't really agree with this

I also want the user to be able specify both the video and the canvas at once, which would end up resizing the canvas on video mutation but not re-positioning it

I never got around to implementing this after I re-wrote this library from scratch, and the whole opts.canvas as of now is just an artifact/placeholder

[ThaUnknown]

46fabb9