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.

mapbox-gl-js/src/source/source_cache.js

Lines 559 to 568 in bf9964e

	if (!tile || (tile.fadeEndTime && tile.fadeEndTime <= browser.now())) continue;
	// if the tile is loaded but still fading in, find parents to cross-fade with it
	const parentTile = this.findLoadedParent(tileID, Math.max(tileID.overscaledZ - SourceCache.maxOverzooming, this._source.minzoom));
	if (parentTile) {
	this._addTile(parentTile.tileID);
	parentsForFading[parentTile.tileID.key] = parentTile.tileID;
	}
	fadingTiles[id] = tileID;

Also related to #4034

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