Fix memory pressure issue with StaticGeometryPerMaterialBatch

We were never assigning the scratch variable we use to store the material in `StaticGeometryPerMaterialBatch`, which caused us to create a new material every frame.
Default StaticGeoemtryPerMaterialBatch material to undefined.
Merge pull request #1622 from AnalyticalGraphicsInc/material-bach-memory

Fix memory pressure issue with StaticGeometryPerMaterialBatch
Merge remote-tracking branch 'origin/master' into dynamicScene-buffering

Conflicts:
	Source/DynamicScene/StaticGeometryPerMaterialBatch.js