Issue Description: When merging the head layers it replaces pixels with semi-translucent ones instead of blending the semi-translucent pixels

• Dynmap Version: core=3.7-SNAPSHOT-949, plugin=3.7-SNAPSHOT-948
• Server Version: Paper 1.21-9-master@4ea696f
• Pastebin of Configuration.txt: I can't access this file, as it is happening on a server I only have OP on.
• Server Host (if applicable): Selfhosted by server owner.
• Pastebin of crashlogs or other relevant logs: None applicable
• Other Relevant Data/Screenshots:
  This is what DynMap generates if going to [DYNMAP_URL]/tiles/faces/32x32/[NAME].png put on top over a red background
  
  This is the same but with a composite made out of the player skin file in an image editor
  
• Steps to Replicate:

1. Have a skin that has the head overlay contain semi-translucent pixels
2. View DynMap
3. See that certain parts of the head are semi-translucent

[ x ] I have looked at all other issues and this is not a duplicate
[ x ] I have been able to replicate this