Additive animation of text x and y with tspan
Two values animations are added with additive sum for
x and y of text ('abcd') containing a tspan ('bc') without any attributes.
The visible result is that the blue 'abcd' (with all glyphs at the same position)
covers exactly the red 'abcd' without any visible animation effect.
If something red gets visible, this indicates an error.
a
b
c
d
abcd