Writer TextBoxes are always wrapped "through", so that they can appear
inside their shapes. However, the surround of the shape may influence
its position. So when surround is asked for anchor position purposes,
take the surround of the TextBox's "parent" shape instead of the one of
the TextBox directly.

With this, the TextBox in the bugdoc is properly positioned inside its
parent shape as expected. (The problem only happens when at least two
shapes are anchored to the same paragraph.)

Change-Id: I0cf8a41080e6759aa395c119d862c4be79574d66
Reviewed-on: https://gerrit.libreoffice.org/23720Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>