Optimizations, Manual Patching and merge with jqPlot fixes.

- Removed unused `var IE_VERSION`;
- `++i` is faster on IE 8-;
- faster clamp (inline `if` > `Math.max/min` on IE8- | jsperf.com);
- very fast `matrixMultiply()` (expanded calculations);
- applied Sencha patch manually (http://dev.sencha.com/playpen/tm/excanvas-patch/);
- manual merge with jqPlot lastest `excanvas.js` fixes (https://bitbucket.org/cleonello/jqplot/src/b233cdeb2e574e323dacd7e54eb379d20aeb3811/src/excanvas.js).