Turkey boss health bar

This fixes #1593 - making the image in the health bar 8-bit.

In addition, I moved the position/style of the health bar so it's similar to the player's HUD and is located in the top right hand corner of the screen rather than in it's fixed position in the floor. I haven't quite finished with the health bar yet - need to make the RHS disappear, will do this later.

I thought perhaps we could make the code for the health bar universal to bosses so the code only appears once rather than in each boss. Obviously this isn't currently an issue as the turkey is the only boss we have. This is beyond my understanding of the code though, so I won't be able to do it myself.