Fewer feature.getAttributes() calls

This PR makes the canvas vector layer renderer perform better by decreasing the number of calls to ol.Feature#getAttributes() per rendering pass. Is is a follow-up PR for https://github.com/openlayers/ol3/pull/1224.

With the [synthetic-data](https://github.com/elemoine/ol3/compare/synthetic-data) example and current master I get the following CPU profile if I constantly zoom in and out using the mousewheel and/or the +/- zoom buttons:

![synthetic-data-profile-master](https://f.cloud.github.com/assets/76594/1490148/530c8742-478c-11e3-9350-5895f9154809.png)

The getAttributes function accounts for a significant percentage of CPU time.

With my style-perf branch the profile looks like this:

![synthetic-data-profile-style-perf](https://f.cloud.github.com/assets/76594/1490188/fe3f4730-478c-11e3-95dc-29e9c5e031df.png)