Added Begin and End methods to View (eventually so that views can support per view render plans)

Signed-off-by: Martin Evans <martindevans@gmail.com>
Added a new type of View "PlanView" which applies a plan when you begin, and reapplies the normal one when it's done. Implemented GBufferTest2.cs which does roughly the same as GBuffer (multiple screens showing different parts of the pipeline) but in a totally different way (an entirely new render pipeline per view).

Signed-off-by: Martin Evans <martindevans@gmail.com>