WIP: Text

WIP. Before merge, need to add tests.

Currently working:

1. Calculation (Cython) and utilization of SDF.

2. Linux text display and system font listing.

3. OSX text display and system font listing.

4. Win32 text display and system font listing.

5. Automatic downloading of a couple nice open-source fonts.

6. Make it a `Visual`.
This was about the simplest I could do based off of Nico's code. We need a way to store the SDF bitmaps in a texture. How could we simplify this?