Highlight labels in demo videos stay inside the picture

Before this change, a highlight's label in a demo video always started at the left edge of its highlight and stayed on one line. A label for a highlight near the edge of the video could end up half outside the picture. Now the label always stays fully inside the video. Near the right edge it moves left until it fits, and if it would go past the top it moves below its highlight. A label wider than the video wraps onto two lines. Videos rendered before this change still show the cut-off label until they are rendered again.