Master issue for Windows redraw performance woes

Currently libui on Windows fakes transparency by drawing the parent control onto each child control as a background each time, resulting in awful performance. Not doing this with the current implementation of uiControl and control hierarchies results in controls indiscriminately having a default background color. I have two options: flatten the control hierarchy or split the work between WM_ERASEBKGND and WM_CTLCOLOR*. @lxn figured a lot of this out.

This issue will combine all existing issues on the matter.

This issue includes and continues the discussion of andlabs/ui#184