minor improvements and code cleanup for canvas.js

These changes are by products of my investigations on slow pdfs.
The impact disappears in the noise, but I still would like to merge them:

* Direct property access using "[foo]" instead of "foo in"
* making FULL_CHUNK_HEIGHT a global variable
* not calculating patternFill every time when read, but set on write
* replaced fullChunk/partialChunk calculation.

What I find interesting is that a (possibly flawed) benchmark for the
floor+ceil code vs the modulo variant shows so dramatic differences:
http://jsperf.com/floor-and-ceil-or-modulo

I had expected that modulo is faster than invoking the methods, but 
the safari 8 result blows my mind. Additionally, I am wondering about
the reason for the Firefox regression for a calculation which has a 
modulo !== 0
The indentation is off here, I would suggest the following instead:
```javascript
maskCtx.fillStyle = isPatternFill ?
                    fillColor.getPattern(maskCtx, this) : fillColor;
```
(The same applies in two places below as well.)