Head overlay is improperly merged if there are semi-translucent pixels

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

webbukkit / dynmap