reduce videosphere radius/segments for perf (#1532)

Author: ngokevin

src/extras/primitives/primitives/a-videosphere.js

@@ -6,9 +6,9 @@ registerPrimitive('a-videosphere', utils.extendDeep({}, getMeshMixin(), {
   defaultComponents: {
     geometry: {
       primitive: 'sphere',
-      radius: 5000,
+      radius: 100,
       segmentsWidth: 64,
-      segmentsHeight: 64
+      segmentsHeight: 20
     },
     material: {
       color: '#FFF',