Outlines with non-opaque blending can draw over line fills

[bcamper]

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See https://github.com/tangrams/tangram-es/issues/502

[bcamper]

Partially resolved by https://github.com/tangrams/tangram/commit/77e436c64225262c80fa67361170e47565574dd2.

Note that we still have issues with outlines drawing on top of intersecting line fills from the same style. For correct ordering, all outlines for the style should be consolidated under all line fills. Fixing this is more difficult, as it's not currently easy for us to place all outlines in the VBO before all fills. A possible future solution is a bucketed draw call architecture, where outlines are tracked as a separate draw call (either in a separate VBO, or a different range in the same VBO as the line fills).