Make sure parent tiles are retained for fading

[anandthakker]

Closes #2467

@mourner The issue was that SourceCache#update() gets called after a tile is loaded, but before drawRasterTile() has had a chance to call tile.registerFadeDuration for the first. since we were using tile.fadeEndTime to decide whether to retain parents/children for fading, the children get removed from the source cache as soon as the tile is loaded.

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs N/A
• post benchmark scores
• manually test the debug page

[anandthakker]

On a plane now, will post benchmarks from the ground

[anandthakker]

benchmark	master 2793b42	2467 7427648
map-load	92 ms	142 ms
style-load	96 ms	88 ms
buffer	929 ms	938 ms
fps	60 fps	60 fps
frame-duration	4.8 ms, 0% > 16ms	4.6 ms, 0% > 16ms
query-point	0.67 ms	0.73 ms
query-box	58.55 ms	61.28 ms
geojson-setdata-small	6 ms	5 ms
geojson-setdata-large	128 ms	125 ms

[anandthakker]

@mourner scores posted, mind giving this a 👀 ?

[mourner]

Looks good.