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?
There is nothing here that forces a recompile, so long as `xform` references the same function every time `draw` is called (and render_transform returns cached transforms, so that should work correctly). However, if the visual has multiple parents (or multiple ancestries), then `xform` will indeed change every time and a recompile will be needed. The solution for that is to let the visual have multiple programs and use a different program for each ancestry. This is a low priority, though, since multiple parenting is not needed immediately.