Clearing the canvas does not work well for time-based (playback) animations

Currently we clear the canvas here: https://github.com/openlayers/ol3/blob/master/src/ol/renderer/canvas/canvastilelayerrenderer.js#L363:L368

However if you're using a WMS-Time based animation, you want to keep the old tiles until the next time step gets in.

Could we provide an option for this? Or are there other ways around this problem? Thoughts?

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