* turns out the clip rect for a window once inside a Begin excludes the
  title bar.  this makes sense, the clip rect is the "client are" of the
  window.
* unfortunately, when a window is docked, we need to draw the texture on
  its tab and when undocked, we have to place it on the classic title
  bar in a slightly different location.
* still messy, but improving.

...i mean, it works, but it isn't as solid as i'd like. gonna think on
it for a while and see if i can come up with any better ideas.