Blender Git Loki

UI: Perf: Use widget base batching

Overall 10% more performance on general UI drawing time.

This commit can introduce ordering problem on some elements.
In this case you need to flush the widget cache to ensure the element that
is going to be drawn is drawn on top of any widget base.

To flush the cache use UI_widgetbase_draw_cache_flush.

This is already done for BLF and Icons.