Build 27: more tiny improvements

What’s new:

• if you are in an area with enemies that are not chasing you, little markers will appear above them so you can find them more easily
• billboards now face behind the player so they’re not rotated to awkwardly when the player is close (eg. dashing into an enemy)
• there is an actual game over message now
• super basic volume controls (press 1/2 to lower/raise the master volume)