Text has point_size and pos properties

And text always draws in that point size, even if inside a viewbox. There are two things to fix, see todo items:

* Is the pointsize correct like this?
* How to prevent recompilation on each draw?
Woops, I set that to linear because nearest made images look ugly. The change snuck into this PR. But let's fix this when we give the Image visual an overhaul (and make a property for interpolation).