Changed VIEW::PrepareTargets() to more appropriate name VIEW::ClearTargets(). Added SELECTION layer to always-on-top list.

Redraws can be requested way too often than it is required. This commit adds redraw timeout:
- if the view became dirty and there has been no redraw for longer than certain time, it is redrawed immediately
- otherwise, we wait for the next frame

This in general improves smoothness of rendering.

More effective way of updating bounding boxes. IsCached() method made public. Removed some of unused fields from the layer description structure.

VIEW_ITEMs save the layer numbers they use, it allowed to speed up removal of items.

Added flipping mode.

Smarter way of the overlay rendering (overlay is always refreshed, while cached&noncached targets only if the viewport or items have changed).

Moved VIEW_ITEM::ViewGetRequiredLayers() functionality to the VIEW class. Now required layers are set per layer basis instead of per item.

Refactored code responsible for high contrast mode. Now it allows to have more than one layer on the top.
Selecting layer using the dropdown list on the toolbar influences the layer displayed in high contrast mode.