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). 
This example working without ```load-pixels``` call. But not workink without ```update-pixels``` call.

This working without ```load-pixels``` because it called internally in https://github.com/processing-js/processing-js/blob/master/processing.js#L19049

But this behavior is not guaranteed in the future. And Processing.js docs says (carefully, a lot of traffic): https://github.com/processing-js/processing-js/blob/master/processing.js#L19116

Cite:
```
* Before accessing this array, the data must loaded with the loadPixels() function. After the array data has
* been modified, the updatePixels() function must be run to update the changes.
```

Note: 
Processing.js use ```PGraphics.imageData``` and ```PImage.imageData``` for buffering ```pixels[]``` array. We can't directly put our pixel values to drawing surface. We must put it to ```imageData``` buffer and call ```updatePixels``` for reloading data in drawing surface.

And (in common case) we must call ```loadPixels``` for reloading ```imageData``` buffer values from drawing surface.

Other ways to working with it is not documented and can be changed in the future I think.