UiMotionCapture

Publishes the UI-animation capture views — the UI analogues of the renderer's motion_vectors pass. capture pass=ui_motion shows WHERE the UI is animating (a change-magnitude heatmap); capture pass=ui_motion_flow shows WHICH WAY it is moving (directional optical flow). Each view's drawing feature is created on demand the first time it is captured, so a session that never selects one pays nothing.