Don't repaint window every frame

[grokys]

We currently repaint the whole window on every frame. This is obviously not ideal - we should just repaint the dirty regions.

[ncarrillo]

I took a stab at this, by having the RenderNeeded stream in the RenderQueueManager pass the transformed bounds of the visual, which then got passed to the Invalidate method of ITopLevelImpl.

I did this and some things seem to work, anything involving dirtying layout is broken, so I need to look into that.

This seems like only half the battle though, as the entire Visual Tree still gets walked and rasterized. It seems like the Paint callback in ITopLevelRenderer passes the top level visual to IRenderTarget.Render. So we need a way to go from the bounds that get passed to the callback to the Visual.

[ncarrillo]

I've done some work on my dirty-region branch here

I mark nodes as dirty on InvalidateVisual and in IRenderTarget.Render I draw only the ones that have been dirtied since the last frame. In ITopLevelRenderer the Paint callback handles clearing the dirty bits from the visuals once done.

There are some rendering artifacts, here and there, and I'm certain the dirty rect getting sent for invalidation is off, but maybe it is a start?