Real-time saveable collaborative drawing

[larryyueli]

User story: the instructor presents the slides using SlideChat during the lecture. When at a certain page of the slides, the instructor "turns on" the collaborate drawing mode. As a result, all students viewing that slide can draw on the slide in real-time. This drawing mode lasts until the instructor turns the drawing mode off. The drawing itself is saved an overlay image for this page of the slide and be viewed later one. Multiple drawings can be saved for the same page, and can be selected from a list of saved drawings.