Speed line shader/effect when boosting

[stbdev]

I've got all the changes pretty much ready locally and can start the PR, one thing however that I would like to merge (if it's ok for you guys ofc) beforehand is the following PR
https://github.com/supertuxkart/stk-code/pull/5888

Whenever you've got a moment, would it be possible to review the PR please? Let me know if I should provide further clarification.