Fix canvas.toDataURL yielding blank image.

[rasmusvhansen]

Added preserveDrawingBuffer as option to getContext for webgl.
Tested in Chrome and IE11

Can be tested as follows:

1. Go to http://phoboslab.org/log/2013/05/mpeg1-video-decoder-in-javascript
2. Run the following snippet in the console
3. Scroll to the bottom and see the blank image.

var img = document.createElement('img');
img.src = player.canvas.toDataURL('image/webp', 0.7)
img.style.border = '1px solid red';
document.body.appendChild(img);

[phoboslab]

Interesting. Thanks for this! However, preserveDrawingBuffer has some performance implications. So I think this should only be enabled through a flag in the options when creating the player, or better yet, through a new method on the player.

I.e. calling player.toDataURL('image/webp', 0.7) would enable preserveDrawingBuffer (or re-create the context!?) and then call player.canvas.toDataURL() internally.

Also, StackOverflow suggests that you don't even need to have preserveDrawingBuffer enabled; the data URL would just need to be captured directly after rendering to the canvas, before the canvas is presented in the browser. Of course this would complicate things when we want to call it on a paused player etc.

[rasmusvhansen]

On a desktop machine, it does not seem to affect performance noticably. I did some rudimentary testing with 8 simultaneous 640x480 streams in Chrome (which is about the limit for smooth streaming on my machine it seems) and the performance seemed to stay the same.
However, the StackOverflow page you linked to also hints at that it will be more of an issue with mobile OS browsers.

I don't know the code well enough to implement enabling/disabling preserveDrawingBuffer dynamically when calling toDataUrl plus I also need it to work for canvas.toBlob.

I have added it as an option to the constructor, so users of JSMPEG can choose whether or not to enable it.
Let me know if that is an acceptable solution.

[phoboslab]

Yep. Since toDataURL() is not the common use-case for this lib, this is a perfectly fine solution. Thanks!