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.
Do you know offhand where it is in the `event` object?

We need it because the `_text_to_vbo` object actually does some setting of the viewport / rendering (since the SDF is computed on the GPU using framebuffers), so it leaves the viewport in the incorrect state afterward. If you ever saw jumpy behavior with the `text` example before (I did), this was the culprit.