Fix 100% cpu when entities using TOSS-based movetypes get stuck
This fixes a bug in sv_gameplayfix_slidemoveprojectiles
which gives nicer
physics interactions but didn't abort if the entity is immovable.
I'll refactor and improve this as part of a larger physics overhaul in a different branch/MR to avoid hectic merge conflicts.
Until that's ready this patch will stop servers from lagging out.
For engine physics this doesn't usually happen because Quake uses sv_gameplayfix_slidemoveprojectiles 0
and Xon no longer uses engine physics, but I'll patch it there as part of some more stuck entity reworking.
Fixes #2774 (closed)