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.
Of course, this was the goal.

Note that since you included the catrom/kernel stuff, they can be used for having the 16 matplotlib texture interpolations for the same price (imshow filter).