Terrain drape texture size doesn't take device pixel ratio into account

Nobody has claimed this yet.

Drape texture is used to render proxy tile in in whole integer zoom range, e.g. [14.00..15.0) for the example below.
Current calculation makes sense only for display with browser.devicePixelRation of 1.

https://github.com/mapbox/mapbox-gl-js/blob/2bf06afbd94b290dabaf1af80f50660cfc68ee39/src/terrain/terrain.js#L368-L370

This needs to be increased on displays with high DPI. The issue is accentuated towards the end of range, e.g. 14.98:

e.g.
http://localhost:9966/debug/terrain-debug.html#14.98/37.753/-122.435

**current 1024** vs **2048** vs **3072** in iPhoneX (browser.devicePixelRatio is **3**).


1024 = `const extent = this.proxySourceCache.getSource().tileSize * 2;`
2048 = `const extent = this.proxySourceCache.getSource().tileSize * 2 * 2;`
1024 = `const extent = this.proxySourceCache.getSource().tileSize * 2 * 3; // browser.devicePixelRatio`

A short performance check shows drop in FPS count on tilted view (30 vs 23 vs 13). Render cache there is disabled - forcing that drape textures are re-rendered every frame. It is also noticeable that the visual difference is not that noticeable (due to bearing):

http://localhost:9966/debug/terrain-debug.html#17.98/37.755668/-122.447788/73.4/70

**current 1024** vs **2048** vs **3072** in iPhoneX (browser.devicePixelRatio is **3**).


Using 2 (2048) looks like an acceptable compromise.

Start at src/terrain/terrain.js lines 368-370 and inspect how the drape texture extent is calculated for fractional zoom ranges. Use debug/terrain-debug.html with the issue's example views to compare texture resolution across device pixel ratios. Done means high-DPI displays use an acceptable extent without an unnecessary performance cost.

Written by the indexing model from the issue text.