ENH: Better text rendering

Implements @rougier's suggestions for sharpening text rendering at large scales, and making it more robust at smaller scales. I also checked, and our size conversion is correct so long as the DPI is correct (currently fixed at 92 in the Text visual).

Ready for review/merge.

Closes #426.
I checked and I can easily use `base64` and `gzip` to save it to disk as 63 KB, let me know if you prefer that.