GroundPrimitive rendering artifacts

Reported on the forum: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/6qt-wb-SohY


This might be either mac or AMD specific. I wasn't able to reproduce this on my machine, but this user saw it on a Macbook Pro with an AMD GPU. It occurs when zooming in and viewing the polygon from certain angles. See the forum post for more details.

```
var viewer = new Cesium.Viewer('cesiumContainer');

var cesiumTerrainProviderMeshes = new Cesium.CesiumTerrainProvider({
url : '//assets.agi.com/stk-terrain/world',
requestWaterMask : true,
requestVertexNormals : true
});
viewer.terrainProvider = cesiumTerrainProviderMeshes;

var pos = [-106.70223205,40.50751048,3108,-106.70231189,40.50719517,3100,-106.7023843,40.50697001,3094,-106.70243232,40.50681935,3090,-106.70250271,40.5066019,3083,-106.70259377,40.50619344,3073,-106.70263983,40.50604864,3069,-106.70268295,40.50579201,3062,-106.70271371,40.50564398,3057,-106.70273965,40.50557274,3053,-106.70276796,40.50531077,3045,-106.70279042,40.50523906,3044,-106.70281277,40.50516771,3043,-106.70285192,40.50504112,3040,-106.70285105,40.50498227,3039,-106.70286683,40.50486993,3038,-106.70288338,40.5047512,3037,-106.70290059,40.50469215,3036,-106.70291193,40.50465355,3036,-106.70292889,40.5045959,3036,-106.70296262,40.50448086,3035,-106.70301364,40.50430164,3035,-106.70304165,40.50420326,3035,-106.70305836,40.50414445,3035,-106.70306945,40.50410539,3035,-106.70309699,40.50400864,3035,-106.703119,40.50393048,3035,-106.70311333,40.50382028,3035,-106.70311326,40.50375577,3035,-106.70310217,40.50366658,3036,-106.70309663,40.5035588,3036,-106.7031076,40.50351989,3036,-106.70310223,40.50347508,3037,-106.70311321,40.50343588,3037,-106.70312416,40.50339683,3037,-106.70314046,40.50333886,3038,-106.70317261,40.50328907,3037,-106.70318312,40.50325271,3037,-106.70323643,40.50319025,3037,-106.70326295,40.50315917,3037,-106.70330982,40.50305671,3037,-106.70333013,40.50298569,3037,-106.70334527,40.50293273,3037,-106.70339025,40.50277541,3037,-106.70340503,40.50272411,3037,-106.70343897,40.50260833,3037,-106.7034534,40.50255908,3037,-106.70347202,40.50249932,3036,-106.70348591,40.50245481,3035,-106.70350443,40.50239484,3034,-106.70351826,40.50235006,3033,-106.7035411,40.50227685,3032,-106.70355447,40.50223604,3031,-106.70349261,40.50221506,3031,-106.70309123,40.50207977,3034,-106.70200464,40.50176103,3036,-106.70087057,40.50172853,3035,-106.700331,40.5018247,3033,-106.69873514,40.50176053,3037,-106.69851119,40.50169164,3038,-106.69836038,40.50152504,3039,-106.69807085,40.50141102,3040,-106.69774118,40.50130369,3041,-106.69750227,40.50120541,3044,-106.69678538,40.50114025,3043,-106.69638718,40.50110116,3041,-106.69560964,40.50088376,3042,-106.69536529,40.50081604,3042,-106.69500127,40.50072198,3040,-106.69484808,40.50068738,3039,-106.69467942,40.50064713,3038,-106.69449011,40.50115032,3036,-106.69434659,40.50147135,3035,-106.6938443,40.50243353,3041,-106.69350654,40.50297144,3051,-106.6929781,40.50382496,3085,-106.69278407,40.50409533,3089,-106.69279539,40.50429847,3090,-106.69274412,40.50472061,3093,-106.69234961,40.50559242,3091,-106.69139427,40.50701375,3079,-106.69111443,40.50740422,3078,-106.69089001,40.50771678,3076,-106.69107519,40.50775148,3077,-106.69135232,40.50780339,3079,-106.69340584,40.50795246,3101,-106.69534761,40.50801909,3124,-106.69626365,40.50793214,3138,-106.69712806,40.50785158,3147,-106.69775128,40.5078214,3146,-106.69790381,40.50783061,3147,-106.6981772,40.50791936,3148,-106.69891842,40.50822559,3149,-106.69924123,40.50836704,3148,-106.6999645,40.50863474,3144,-106.700285,40.50866932,3141,-106.70037259,40.50867131,3141,-106.70042119,40.50865747,3141,-106.70074046,40.50862013,3139,-106.70090489,40.50865367,3137,-106.70128863,40.50868389,3135,-106.70141291,40.50870639,3133,-106.70162049,40.50874742,3130,-106.70166649,40.50874076,3130,-106.70173136,40.50874534,3128,-106.70179559,40.50875389,3127,-106.7018538,40.50877897,3127,-106.70191239,40.50880195,3126,-106.70199749,40.50880949,3125,-106.70206303,40.5088092,3124,-106.70207731,40.50875908,3124,-106.70208575,40.50865554,3123,-106.70208719,40.50857644,3122,-106.70206234,40.50851386,3121,-106.70204609,40.50834978,3120,-106.70202152,40.50828782,3119,-106.70201622,40.50823352,3119,-106.7020374,40.50816088,3118,-106.70203956,40.50808012,3117,-106.70206062,40.50800797,3117,-106.70207459,40.50796008,3116,-106.70208849,40.50791261,3116,-106.70210864,40.50784577,3115,-106.70213527,40.50775779,3113,-106.70214837,40.50771513,3112,-106.70216142,40.50767258,3112,-106.70217444,40.50763016,3111,-106.7021939,40.50756674,3109,-106.70220682,40.50752462,3109];

var myGeom = new Cesium.GeometryInstance({
geometry: new Cesium.PolygonGeometry({
polygonHierarchy: {
positions: Cesium.Cartesian3.fromDegreesArrayHeights(pos)
}
}),
attributes: {
color: Cesium.ColorGeometryInstanceAttribute.fromColor(new Cesium.Color(1.0, 0.0, 0.0, 0.4))
}
});

viewer.scene.groundPrimitives.add(new Cesium.GroundPrimitive({
geometryInstance: myGeom
}));

viewer.scene.camera.setView({destination: Cesium.Cartesian3.fromDegrees(-106.7022, 40.5075, 10000)});
```

Start with the provided Cesium Viewer example, especially CesiumTerrainProvider, PolygonGeometry, and viewer.scene.groundPrimitives.add. Reproduce the artifacts while zooming and changing viewing angles, using the linked forum report for additional context and testing on the reported Mac/AMD setup. Done means the supplied GroundPrimitive polygon renders without the reported artifacts.

Written by the indexing model from the issue text.