MSDF Texture Atlas

[corbanbrook]

The texture atlas will store the MSDF bitmaps created by the MSDF generator.

Will need to support some sort of async box packing algorithm which would allow us to add/reserve new rects from the atlas based on glyph data. Most box packing algorithms have access to all rect data upfront in order to find the most optimal solution. Adding rects over time will be suboptimal but we only want to reserve texture space as new unique glyphs are needed to be rendered.