Font serialization and 2D text drawing

Basic implementation of 2D text

In the assets convertor you can convert some fonts

For drawing text you have to call

`getInstance<FontManager>()->draw2DString("My text", "myFontName", 40, glm::ivec2(10, 10), glm::vec4(1), "2DText");`

text, fontname, size, position, color, shadername

see `BulletDemoScene.hpp` for example

implementation is naive and can be largely improved