Fix icon rendering on hidpi devices

This PR fixes a major regression introduced with https://github.com/openlayers/ol3/pull/2040 (icon sprite support) where icons (and circle symbols) are either not rendered with the expected size or not rendered at all on retina devices. This was reported on the [mailing list](https://groups.google.com/d/msg/ol3-dev/vtT-3IRCAjc/iGcHfaSTTQIJ) and in an old GitHub [issue](https://github.com/openlayers/ol3/issues/1918#issuecomment-42630958).

The fix involves using the icon/symbol image's width and height as the width and height of the source image in the `drawImage` call. Previously, values multiplied by pixelRatio were passed, which was wrong.

Please review.