Reduce blurriness of draped line rendering

[mpulkki-mapbox]

Prior to this change lines were appearing too blurry in draped rendering (terrain and globe) because wrong pixel ratio was used on high DPI devices. The ratio should always be 1 when rendering to an offscreen texture. Too large pixel ratio will mess up the anti-aliasing logic resulting in this pixelated/blurred effect. Ported from gl-native, original author @endanke.

Before and after images from an iphone XS:

TODO

• Add render tests to validate the change

Launch Checklist

• briefly describe the changes in this PR
• include before/after visuals or gifs if this PR includes visual changes
• write tests for all new functionality
• manually test the debug page
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Reduce blurriness of line layers when used with the terrain on high DPI devices.</changelog>

[karimnaaji]

Looks good to me!