Add widget 3d transforms with nesting?

[lamarrr]

Things to think about

• Mouse interaction
• Z indexes in vertex3d
• Nesting and relative positioning to maintain transforms
• Transformation of pointer to widget space using quads?
• How well will this work in a 3d context?
• Widget::draw should probably be relative to the widget's position by default???
• We'll need clip transforms, Canvas 2d to 3d transform??