LineSymbol: Fix dashed line with mid symbols

[dl3sdo]

Dashed lines with mid symbols that are placed in the center of a dash are sometimes disturbed. This commit ensures that the dash is always drawn the same way, independent of whether mid symbols are present or not. Fixes #2064

[dl3sdo]

@dg0yt @lpechacek: Do you prefer correcting commits (and then squash when merging) or should I force-push instead?

[lpechacek]

@dg0yt @lpechacek: Do you prefer correcting commits (and then squash when merging) or should I force-push instead?

As far as I'm concerned, squash on merge might be relevant in this particular case. In any case, Kai is the current code repo owner, so the final decision is with him.

[dl3sdo]

The check was aborted due to https://github.com/actions/virtual-environments/issues/6002 Thus I intend to squash and force-push in order to have a successfully checked PR.