Optimizations, better IE8- detection, faster matrixMultiply, Sencha Patch, jqPlot ExCanvas merge, minor fixes and opts.

- 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);
- `++i` is faster on `IE8-`;
- multiply is faster than divide on `IE8-`;
- `Math` call optimization;
- very fast `matrixMultiply()` (expanded calculations and cached common calls);
- only `IE8-` initializes `VML`;
- `window`, `document` and `undefined` cached for performance and better minification;
- minor changes.