Mouse movements can cause whiteboard image to be erased on the server

[dmnisson]

Related issues: #296

If the user reloads the page and the mouse is in the whiteboard canvas, and it is moved out before the image loads, the image data is erased on the server so that when either user reloads the page, the whiteboard becomes blank.

Steps to reproduce:

1. Have a student create a new session, and have a volunteer join.
2. Have the student draw on the whiteboard, then refresh the page and move the mouse cursor into the area of the whiteboard canvas while it is loading.
3. As soon as the whiteboard appears, but before the image appears on the whiteboard, have the student move the mouse out of the canvas.
4. Without moving the mouse back into the whiteboard canvas, refresh the page to see that the whiteboard is now blank. (This step is not always necessary to reproduce the issue--sometimes simply reloading the page and then moving the mouse out of the whiteboard is enough.)

These steps are easiest to perform on a slow connection.