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`.
That's actually not true in my implementation, and I'm not sure why it would be the case generally. Surely a "W" SDF should be much larger than a "." SDF...?