Add italic & bold

[felixmariotto]

Mentioned in #151, we decided to handle this in a separate milestone.

The idea is to let the user pass font files for italic / bold and set the font as italic or bold via an attribute.

In case the user didn't provide font files for bold and italic, we would revert to shader-based solutions:

• for italic, slent the character quads in vertex shader
• for bold, using different values on distance field in fragment shader

This issue should be addressed after #153 is addressed and 7.x.x is shipped (in particular the refactoring of MaterialManager)