UI and transform performance updates

[BrentFarris]

I've made a few updates to improve the performance and use goroutines a bit more for concurrent work. The first update was to the UI system. I completely overhauled the UI so that there is no longer a UI interface but instead a singular UI struct that all UI elements are. This allows all different UI elements to be packed into a linear array and pooled. Each UI element has a block of data unique to it's own element type, but the rest is shared. I've also added pooling and a UI Manager which uses pooling to manage a linear array of raw UI structures. The second performance update was to Transforms. Now they are not looped through in the host, but instead work for updating and resetting their dirty state is done in a WorkGroup. This is a concurrent grouping of work that executes and blocks until all work is complete.