Batch draw calls in GUI renderer

[benreid24]

Changes are in the gui-draw-batching branch. Current status:

• Draw calls for Box are batched

• Window and Box provide batch layers

• Some issues with draw ordering. In particular background items seep through windows and text outlines

• Mouse highlights are mostly broken

• Scroll area scissor is not respected

  • Was thinking of adding a new scissor type to OverlayScaler that is a manual scissor, but don't think that is the root cause
  • May have to do with where the container box is packed, or more render order + depth issues from above

• [x] Default Box to not constrain view by default

• [x] Add intermediate base class for batch shape component hosts

• [ ] Update batch components to use it

  • [x] Box (force batch when view is constrained, delegate to parent otherwise)
  • [ ] Button
  • [ ] Icon
  • [ ] ProgressBar
  • [ ] Separator
  • [ ] Ensure that elements using Box continue to work once batching is added:
  • [x] Window
  • [ ] Notebook
  • [ ] ScrollArea
  • [x] GUI