[text] Add .negate property for msdfgen >1.4

[donmccurdy]

Fixes #3382.

Testing with fonts created here: http://msdf-bmfont.donmccurdy.com/

text="align: center; width: 6;
    font: /examples/assets/fonts/yahei.json;
    value: 你好，世界;
    negate: false;
    shader: msdf;"

[dmarcos]

Thanks!

[machenmusik]

Nice! Maybe we should regenerate the older default font with the new tool, and default negate to false (at least for msdf) so all new msdf fonts we create will work without the flag?

[donmccurdy]

The fonts in aframe-fonts would need to be updated too, they are generated from https://github.com/Jam3/msdf-bmfont, which is still using the old msdfgen version. Users reporting issues because of that: etiennepinchon/aframe-fonts#1 (comment)

I'm hoping my little web app can help out here, and let you generate just the character set you need (especially necessary for chinese users), but i need to iron out some issues first. So, probably won't want to change the default for 0.8.0 I don't think.