Text has point_size and pos properties

And text always draws in that point size, even if inside a viewbox.
Text: can set color, and supply pos and point_size in init
ModularShader: Fix that program is not recompiled on prepare()

... unless necessary
Address comments
Text: address comments
flake
Text.pos does not use transform.
Text usable in normal canvas
Text: addressed comments
Text: fix scale when Text not used in SceneCanvas.

This should fix the tests
flake-arg
Merge pull request #425 from almarklein/text-improve

Text has point_size and pos properties