Fix set-pixel and get-pixel functions

I think that adding support of ```pixels[]``` to ClojureScript version don't have a sense.
Because user must pack\unpack color to ```pixels[]```:

https://github.com/processing-js/processing-js/blob/master/processing.js#L19153
https://github.com/processing-js/processing-js/blob/master/processing.js#L19136

So, in this case I think that users will use ```set-pixel``` and ```get-pixel``` for working with ```pixels[]```.
Because realization of ```pixels[]``` in Java and JavaScript Processing is different.


P.S. Tests for ```set-pixel``` and ```get-pixel``` maked manual. Because if we run it in common canvas this is raise Security exception (canvas context dirtied by previously loaded images). 
We don't access `imageData` in `set-pixel` directly so I doubt we need to call `update-pixels`. Actually `updatePixels` called automatically in `resetContext`: https://github.com/processing-js/processing-js/blob/master/processing.js#L19054

As I understand `loadPixels` and `updatePixels` should be used only when you access `pixels` array directly. It's not the case for `set-pixel`. we don't access the array, we just call method.