Small Fixes & Less Bloodstone Loss

A small patch with bug fixes and some difficulty adjustments: You'll lose now only 50% of bloodstones as default and can decrease it to 0% with upgrades.

• Fixed a bug making Sewers-boss harder
  
• Fixed a spelling error
  
• Decreased bloodstone loss
  
• Hires sources for large bg art
  
• Limit control to relevant player in item picker, when only using gamepads
  
• Immunities for enemies in endless challenge
  
• Lowered doom skull & death blow limits to 8"