Implement moving, fix util.array bug in opApply. Also:
- Make the shared tastCount variable a byte array in gui.rtdemo
(so that each task type can have independant element for synchronization).
- Add initial value for rt.color (because nan make toRGB32(..) very slow).
- Refactor rt.renderer (enable prepass drawing, return after it
if in interactive mode).
- Enable interactive mode in data/lecture5.sdl