Merge branch 'sev/buff_updates' into 'Mario/buff_updates'

Buff revisions

This branch combines all my visual buff updates.

- The buff shaders have been revised for a consistent quality
- Buff icons are added to the HUD themes
- The powerup panel now also shows the buffs with their icons
- The buff colors have been updated

One thing that is left: The buff HUD panel, and all associated cvars, are now obsolete.
If you agree, I can remove them.

See merge request !160