Add multichannel SDF support

[PixelsCommander]

Multichannel SDF provides better quality for text rendering. Better explained here
https://github.com/Jam3/three-bmfont-text/blob/master/docs/sdf.md

In order to use MSDF we should modify fragment shader (src/fragment) and have a corresponding demo based on font generated with: https://github.com/Jam3/msdf-bmfont

Here is a one we may use https://github.com/Jam3/three-bmfont-text/blob/master/shaders/msdf.js

[cpietsch]

any news on this topic ? would love to use MSDF with pixi!