When I implemented the no preview for span I did originally try using text in
the canvas, but discarded it when I realised trying to format it to display
nicely (across different languages and text lengths) was a losing proposition.
However just the prohibition symbol without context is confusing to users, and
on reflection the text can easily be included as an element stacked on top of
the canvas.
Also lighten the background of the no preview message, since the pure black was
indistinguishable from the frame sides on Windows and Linux.
Differential Revision: https://phabricator.services.mozilla.com/D29750
f9cede0ea2