line-round-limit greater than 1 causes rendering errors on iPhone 6

[jfirebaugh]

See #5234. The issue appears with Mapbox Streets on iPhone 6 (but not any iOS simulator and not the OS X SDK). No interaction is required, other than moving to the pictured area:

This was exposed when the default value for line-round-limit was changed from 1 to 1.05, to obey the style specification. As a workaround, we have added an override that forces the default back to 1. But presumably any style that explicitly specifies "line-round-limit": 1.05 will still have rendering artifacts. We need to find the root cause.

[stale]

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