fix the transparency gif in pull request

[Jandou6]

I have a pull request that fix the transparency Bug.
I hope that author can see it.

[Jandou6]

#77 in here

[rustinqi]

This PR really saved my day! Thanks and hope it can be merged soon.

[programmerC]

hey guy,
I quoted your gif.js and gif.worker.js and then I am getting an error.
ctx.getImageData is not a function
at GIF.getContextData (gif.js:2)
at GIF.getTask (gif.js:2)
at GIF.renderNextFrame (gif.js:2)
at GIF.render (gif.js:2)

here is my code:
var gif = new GIF({
workers: 4,
quality: 10,
background: '#000',
transparent: 'rgba(0,0,0,0)'
});
gif.addFrame(canvas, {
delay: 200,
copy: true
});

[nicolasdelfino]

@programmerC @SouthpawGoblin Has one of you managed to get this to work?