Text on path: remove optimisation, allow tspan to be kept

When all text on path is deleted, keeps the empty tspan(s), so alignment can be remembered. Useful when deleting all the text, then changing it to something else. This copies behaviour when not on path.

This can/will lead to a proliferation of empty tspans and the like, but will be isolated to text on path.

out left: old behaviour, right: new behaviour

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Other commit is a standard out of bounds check for flatpak/debug build.
Fixes #261 (closed)

Odd, couldn't rebase using the gitlab interface just now 😕