Avoid caching mImage on the main render loop and use clipping.

+ Added a session flag that avoids caching mImage. This is useful when
mImage is just a Graphics2D wrapper that might change at any time.
+ Make GcSnapshot aware of the clipping so it doesn't need to render the
whole area if not needed.

Change-Id: Ie0134c2bc2741b5fa6648313864c851bbac066cf