Prefix internal classes properly and private export them.

Without the private export on these classes, the private
export on the QSGDefaultImageNode and QSGDefaultTextureNode
is useless, as the symbols fail to resolve

Change-Id: I25a265a7669e5f5015c482aa24fe154c2c9a1062
Reviewed-by: Alan Alpert <aalpert@blackberry.com>