Raster tiles are always retained when `raster-fade-duration` is set to `0`

[stepankuzmin]

GL JS always retains raster tiles if raster-fade-duration is set to 0.

It's most likely related to how we handle fading tiles in SourceCache.

https://github.com/mapbox/mapbox-gl-js/blob/bf9964eba975ed2f82294c1d906787a93050cfff/src/source/source_cache.js#L559-L568

Steps to Trigger Behavior

1. Add raster source
2. Set raster-fade-duration to 0
3. Zoom out and see that children tiles are still retained

Link to Demonstration

https://codepen.io/stepankuzmin/pen/zYjNQWM

Expected Behavior

Parent tiles should replace children tiles

Actual Behavior

Children tiles are retained, parent tiles aren't drawn