When inserting characters with no font glyph match cursor positioning incorrect

[rocketraman]

If characters are inserted for which the Opentype font has no glyph, the code assigns a default width of 0 to the character. This causes the line wrapping and cursor positioning logic to be incorrect.

Instead of assigning a default width of 0, fallback to a canvas-based measurement to see the width the browser will use when rendering the character.