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.
64 KB is too big to include in the module. so I guess we need to put it in a data file. It is taken into account during installation, I assume? If its 63 KB with base64, then a simple gzipped blob would be about 47 KB so maybe just leave it as it is.