Complete transform support

[shans]

Still to do:

• promote 2d animations to 3d animations when interpolating from 2d to 3d or vice versa
• implement special rules for 3d rotation
• clarify whether rotateZ should collapse to rotate in 2D case
• clarify whether rotateX-Z should collapse to rotate3d
• implement special rules for matrix interpolation
• implement special rules for perspective interpolation