Spritemaps: Put placeholder at coordinates (0,0)

[dbrgn]

The current spritemaps all have a character at position (0, 0). This is problematic when adding CSS classes for an icon that does not exist.

For example, here I tried to create an emoji picker but made a mistake in some of the codepoint classes:

This results in the first emoji being shown, because there is no offset as defined by the appropriate codepoint class.

While this example is a bit contrived, we had the problem in Threema Web that newer emojis sent by iOS devices would be rendered as policeman because the appropriate emoji was missing:

threema-ch/threema-web#187
threema-ch/threema-web#194
threema-ch/threema-web#199

(The reason was a modifier that was not supported in emojione 2.0...)

To alleviate this issue and to prevent problems with the wrong emoji showing, it would be better to put a placeholder at position (0, 0) of the spritemap, e.g. a light grey square, a box with a quesiton mark, something like that.

[kevinranks]

I like this idea, and we'll add it to our backlog for a future release. Thanks for the input!