Previously we were using CSS to make the text's colour transparent.
However that proved to cause a lot of problems with pre-existing
transition animations in the host webpage. There didn't seem to be
anyway to disable the transition time for text transitioning to
transparent, without also disabling all transitions.
Also added censorship to password input boxes.
Using JS's `getComputedStyle()` for every character is too CPU
intensive, so instead I'm experimenting with using a custom font
to take the canvas snapshot. The font is made up of only the unicode
block character, which basically fills the entire space given to a
monospace glyph. This also means that we can fairly reliably work out
the visibility (whether it's obscured or hidden with CSS) of text.
