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.
Sure. It also occurred to me that you'll need to do it this way if not using a `SceneCanvas`. (It's also highly unlikely to be the primary bottleneck since SDF calculation isn't that fast.)